I’ve taken my own stock countless times. Many times, I was baffled. Over time, I’ve accumulated knowledge about situations in which this takes place. For the purpose of this list, something that is not technically an SD but that results in an easily avoidable death counts as an SD. Here’s a list that is roughly ordered in terms of most common to least common:

Trying to tech and then getting hit before you hit the ground will make you miss the subsequent tech, which can easily be followed up on with a knee, fox usmash, etc. Be calculated every time you full press a trigger because it creates a significant window where you cannot tech (if I recall correctly, it’s a 20 frame window where tech is buffered followed by a 40 frame window where you cannot input another tech).

Hard pressing your trigger when you L-cancel will result in you subsequently missing the tech if you get hit out of your aerial because of this lockout mechanic. L-cancel with light press or even Z.

Trying to react to getting hit with a tech input can result in an air dodge if you are not cognizant of your DI because you might slide off the stage as you go for the tech input.

When you grab the ledge, let your analog stick reset to neutral before doing an input on the analog stick. If you try to ledgehop without letting your stick reset, you will tournament winner, which will likely be punished and result in loss of a stock. Similarly, if you don’t think you are going to reach the ledge and try to up+b, but then do end up grabbing the ledge before your up+b, this will probably result in a tournament winner.

If you roll to the edge of the stage, your ECB is in a state such that you must do a frame perfect wavedash. Depending on how egregious your timing is with the wavedash, failure to do a frame perfect wavedash is likely to result in you airdodging off the stage.

Going from light shield to full shield has at least one frame in which you are not at all protected by your shield. If you want to full shield, make sure to depress the trigger all the way immediately.

Attempting to jump as you dash or get pushed off the stage can result in a ledge cancelled jump squat. Don’t ledge cancel your jump squat and then do an aerial and fall to your death.

Attempting to shield DI can result in very bad regular DI. It’s difficult to learn exactly how each move can be used to shield poke you, but it’s important to try to do so. Using light shield can help mitigate this problem. The most common situation where this is costly is when people are angling their shields horizontally to prepare to shield drop. Getting hit while doing this will result in abysmal DI. For some characters like Marth, who are easier to shield poke while on the ground, you will have to be extra mindful of how you might get hit out of your shield and be ready to DI those moves.

If you attempt to SDI but your opponent does not time their move as you expected, remember to still input regular DI after your SDI input. Sometimes people get too fixated on SDI and then miss regular DI, which might be enough to survive.

If you attempt to buffer an action using the c-stick, make sure to not do the c-stick input on the first frame you are actionable. If you tried to buffer a roll and got an fsmash, this is because when the c-stick input is on the same frame you become actionable, the c-stick input takes priority over the attempted buffer roll input.

Accidentally buffering a jump out of hitstun by hitting up on the joystick from DI or SDI is an easy way to throw away your stock. Be cognizant of exactly how long you are in hitstun so you don’t do this by accident or let your opponent trick you into doing it in high hitstun situations.

Accidentally buffering a getup attack by getting hit while you’re pressing buttons is really bad, especially in doubles. Don’t spam buttons excessively—know exactly how each input is going to map onto the screen.

Spamming jump, up+b, or side+b to recover as quickly as possible results in you recovering slower than timing the input. The best mashers in the world can get about 15 mashes per second. This means that, if you are in the top one percentile of mashers, you still can only get one press per every four frames. For frame tight situations, you really need to learn when your stun/lag ends and time your button presses accordingly. An effective strategy for mitigating this learning curve in situations where you have your jump is to press up on the joystick, then both x/y right as you think you’ll be actionable because up on the joystick gives a small buffer window for jumping, and the x/y presses will come out very shortly thereafter in case you tried to buffer jump too early.

Air dodge is not frame one. Often when people try to air dodge onto the stage, they do it too late and get hit before the air dodge comes out. An easy way to tell if this was the case when you tried to airdodge is to enable V-cancel sound effect on the 20xx hackpack. A slightly late air dodge will result in getting a V-cancel.

Failing to properly account for hitlag of offstage moves is likely to result in a botched recovery. Hitlag on average is three frames, which means that if you hit your attack off stage, you should input your recovery action three frames later if the move hits than if it does not. Electric moves have double hitlag, so hitting with an electric move requires waiting longer (note that this makes it particularly important for Falcon mains to recognize if they are going to hit with weak knee, strong knee, or whiff).

IASA frames cannot be interrupted by B moves. If you try to jump out of an aerial off stage, you can generally jump earlier than you can up+b. If you are going for an up+b or side+b out of your aerial, be cognizant that you will probably have to wait a few additional frames compared to if you had jumped out.

Learn which moves contort your ECB to avoid situations where you try to jump back on stage with an attack, only to miss the stage and fall to your death. There are often times where you will land on the stage if you don’t throw out an aerial, but doing the move alters your ECB such that you don’t make it on the stage.

Hitting people off platforms, both when they are vulnerable or when shielding, can result in you getting counterattacked, particularly in this era of ASDI down. Light shielding at the edge of a platform is many years old, but some people are still not used to ASDI down. Don’t hit someone off the platform only to lose your stock to an easy counter attack.

Not wiggling out of tumble can result in significant windows where you can get punished in a situation where you’d likely be safe. Wiggling out can be done simply with one movement of the analog stick from center to either side, so you should try to time it. Note that a “bad dashback” also results in a failed wiggle, hence why many top players still opt to wiggle instead of doing only one movement of the stick.

If you Amsah tech at high percent, be sure to fast fall to the ledge or use the 1 frame input trick to break your momentum to avoid flying off the stage.

When shined by Fox near the ledge, be sure to fast fall to the ledge. Most often you also want to DI or even SDI away on the shine before the fast fall in order to get to the ledge as fast as possible. If you DI in, you might get shined a second time before you snap to the ledge. Input your fast fall later if you get double or multishined. *When shined at the edge of Yoshi’s, if you play a character that does not get knocked down by shine and ASDI down, you’ll land and be actionable. Use this to your advantage and don’t do the fastfall input.

If you fall off a platform with your back to the edge of it while shielding, input a fastfall in order to avoid having to tech. Sometimes people will even intentionally push you off a platform, and in these cases, fastfalling is a crucial piece of counterplay.

If you are near the ledge, be mindful that you might get pushed off the stage. Many things can make this happen. Don’t let someone get a kill by rolling into you like this https://www.youtube.com/watch?v=9-HaDBU1PRc&t=7m15s. Other common situations include getting pushed off from doing a powershield, a ground move clanking, or from someone doing a ledge standup.

Holding forward on the analog stick while recovering on Battlefield is usually what causes you to get “Battlefielded.”

Randall makes people mess up all the time. Learn the timer. It comes out when the 1s digit is a 5 (technically right after it turns to 4). When the 10s digit is even, it’s on the right; when it’s odd, it’s on the left. Thus, when the clock reads 7:55:00, it’s about to come out on the right. Remember that on the right side it comes out low and on the left side it comes out high. Don’t let it come out and ruin your ledgedash on the left side.

If you try to up+b, then unexpectedly land on Randall, you are extremely likely to side+b. This is because a significant amount of the area on the analog stick that gets read as up+b while in the air is mapped to side+b when grounded. In these situations, make sure your joystick is pointed near 12 o’clock in order to avoid an accidental side+b.

If you get a bair input when you tried to Scar jump, this is most likely because your c-stick was moved after your drop from ledge input before it reset to neutral. You don’t need to be delicate when it comes to holding away from the ledge with c-stick. What you need to do is make sure it gets back to neutral without moving vertically at all. Note that this can be highly variable between controllers.

Wispy always blows towards the edge of the stage. Sometimes characters with high amounts of landing lag will land on the platform. In these cases, you might be used to milking all of their lag to punish them. If you do this, they might get under you and punish you.

Sheik is actionable much sooner when she clips the edge of the stage with her up+b. Don’t mess up your punish and then get shieldgrabbed or spotdodge grabbed into a gimp. See this thread for details https://smashboards.com/threads/sheik-up-b-mechanics.424253/.

If you try to roll from the edge while ledge guarding Luigi, be sure to use light press to roll. People die far too often from airdodging as they try to roll from the ledge, which will not happen if you light press the trigger.

When trying to teeter cancel, letting go of the joystick too early makes you run off the stage. Go into your run animation, then release the stick as you get closer to the ledge. This seems counterintuitive, but teeter canceling is very easy, so practice it for a couple minutes and you should get it down.

If you try to approach with grab and buffer dthrow, then get hit out of your approach, you might buffer a spot dodge.

Often times, when you’re in lag, perhaps from doing an up+b onto the stage, the opponent has only a small set of options that can end your stock without any mixups. In these situations, it’s possible that you have a defensive option—probably involving ASDI or SDI—that will prevent them from being able to end your stock. In these situations, you should always do the input just in case, because you’re in lag, so it won’t result in anything else coming out. For example, when I whiff a raptor boost at high percent against Marth, I always input an Amsah tech because they are likely to go for up+b or fsmash. Near the edge of the stage, they may opt to grab me instead since I can’t Amsah tech that, but then I can react to the grab. Sometimes after doing an up+b onto the stage, Marth players will opt for ledgehop nair because that’s his aerial with the most knockback. In these situations, you should always hold down, because then you can shield before the second hit. If Peach grabs you at high percent, immediately DI for fthrow and react if she does another throw. Fox going for ledgehop drill into waveshine usmash is also a common situation which can be countered, as it’s easy to SDI, but this isn’t quite the same since it’s likely the Fox player will have additonal kill setups. Learn commonly occurring situations where you have defensive counterplay that comes at no opportunity cost!

When doing an up+b onto the stage as Falcon, Ganon, or Marth on Yoshi’s or Stadium, if you land on the last pixel of the stage horizontally, you’ll fall off the stage. This tends to actually be a good thing, but if you are unprepared, you’ll probably fall to your death.

If you’re on the left side of the rock transformation, many grounded options will push you off the stage. Test which moves do this so you can know which ones to use should you get into a weird situation down there.

If you fall through Stadium, try to recover. On top of saving your stock, you might even look like a badass like this https://www.youtube.com/watch?v=5BkxCjohS9Y&t=4m40s.