Some spoilers ahead for Avengers: Endgame.

Because Avengers: Endgame is a movie about time travel, it has a scene in which the Avengers rattle off a list of time travel movies.

There's Back to the Future, of course. And Star Trek, Terminator, A Wrinkle in Time, Bill & Ted's Excellent Adventure, Time Cop, Hot Tub Time Machine —

Wait. Hot Tub Time Machine?

Starring this guy?

Also familiar to Marvel fans as this guy?

Yep, Avengers: Endgame, a movie starring Sebastian Stan, has a reference to Hot Tub Time Machine, another movie starring Sebastian Stan. Which opens up a whole new can of worms for the Marvel Cinematic Universe, which is basically all cans of worms already.

Does Sebastian Stan, the actor, exist in a continuity where Bucky Barnes, a man who is exactly identical to Sebastian Stan, also exists and enjoys a thriving acting career? Or are we to presume that in the MCU version of Hot Tub Time Machine, the Stan role (Blaine) is played by someone else?

None of them got back to me, which I totally get, but which did not help me sleep any better at night.

Does all the pop culture we have in our universe appear in theirs, or only a narratively relevant select few? (I mean, the Marvel movies can't possibly exist in the same form over there, right?)

To be sure, this isn't the first time an MCU movie has referenced a non-MCU project starring an MCU actor. Endgame itself has Tony making a reference to The Big Lebowski, starring Jeff Bridges, whom you'd think Tony would notice is a dead ringer for Obadiah Stane, his mentor-turned-nemesis from Iron Man.

In addition, there are references in Captain America: The Winter Soldier to Star Wars (starring Nick Fury actor Samuel L. Jackson and Jane Foster actor Natalie Portman) and Star Trek (starring Thor actor Chris Hemsworth).

But it's possible to hand-wave away some of these overlaps. Winter Soldier confirms only that Star Wars and Star Trek exist, not which specific films within those series. For all we know, the MCU never got the Star Wars prequels or the J.J. Abrams Star Trek movies.

Okay, so the Lebowski nod is harder to explain away — but Iron Man was so long ago, which makes it easier to forget Bridges was ever part of this world at all.

Endgame, on the other hand, seems to be (and I'm sure you nerds will correct me if I'm wrong) the first MCU film with an unambiguous reference to one of its own actors' other movies.

The actor Sebastian Stan as he exists in our universe: blissfully unaware that he might not exist in the Marvel Cinematic Universe. Image: Alberto E. Rodriguez / Getty Images for Disney

Rattled by this discovery, I reached out to Disney, and the agents of Endgame directors Joe and Anthony Russo, screenwriters Christopher Markus and Stephen McFeely, and star Sebastian Stan to ask for clarifications. None of them got back to me, which I totally get — I probably wouldn't have gotten back to me either. But the deafening silence did not help me sleep any better at night.

After all, this isn't a problem that's just going to go away as the MCU continues. It's one that'll only get worse, as the franchise pulls more and more actors into its orbit.

Already, there's a decent chance that any recent-ish English-language movie you turn on will feature at least one actor from the MCU, whether it's '60s classic Butch Cassidy and the Sundance Kid (starring Robert Redford of Captain America: The Winter Soldier) or 2018 Oscar winner Green Book (with Linda Cardellini of Avengers: Age of Ultron).

So, left to my own devices, here are my best guesses for what the hell might be going on here:

It's a Last Action Hero situation

At one point in Arnold Schwarzenegger's The Last Action Hero, the characters encounter a poster for Teminator 2. Except in their universe, the film stars Sylvester Stallone.

It seems at least worth considering that this is what's happening in the MCU — that they have all the same films we do, only with different actors in them. Maybe their version of Clueless stars, I don't know, Ben Affleck, or their Mean Girls has Katherine Heigl. I bet Chris Pine gets all the Hemsworth / Evans / Pratt roles in that continuity.

After all, it's not like Rhodes references Stan directly in his Hot Tub Time Machine shoutout. It's possible to shrug it off and assume that in their timeline, the role of Blaine went to Dominic Cooper Jake Gyllenhaal Zachary Levi Ashton Kutcher.

And perhaps Stan, the performer, doesn't exist at all. It'd be a shame for the moviegoers of the MCU, since Stan is a really good actor, as evidenced by films like, well, the Captain America movies. But they won't know what they're missing anyway.

They're going the Ocean's Twelve route

In Ocean's Twelve, Julia Roberts plays Tess Ocean, who in her own reality is just a regular person who happens to look exactly like Julia Roberts, and whom she therefore impersonates for fun and profit.

Is it possible, then, that Avengers: Endgame unfolds in a world where Bucky Barnes simply looks identical to a totally unrelated actor named Sebastian Stan? Seems like a pretty messed up situation for a guy who can barely remember his own past as it is, but what can you do?

There is one big clue pointing to this as the most plausible explanation for how the actor Sebastian Stan can coexist in the same reality as Bucky Barnes, who in our reality is played by Stan.

Stan Lee has appeared in every single MCU movie in different role. He's been a World War II general in the 1940 (Captain America: The First Avenger), a World War II veteran in the 2010s (Avengers: Age of Ultron), a FedEx delivery man (Captain America: Civil War), a bartender (Ant-Man), and even himself in the 1990s (Captain Marvel).

There's a theory that all along, he's been playing a single character — that of Uatu the Watcher, as was hinted at in Guardians of the Galaxy Vol. 2. But the point here is that there's just this one guy who keeps popping up all through space and time, with a Forrest Gump-like knack for being in the right place at the right moment, and no one seems to have noticed or found it odd.

Thus, we can conclude that the MCU is a world in which it's just not that weird for a bunch of people to run around with the exact same face.

So Bucky Barnes looks exactly like movie star Sebastian Stan, and it's not a big deal. Thor thinks it's funny that people keep asking him if he's Chris Hemsworth, star of the Ghostbusters reboot. Tony Stark and Robert Downey Jr. run into each other at swanky Hollywood to-dos and snark about each other's facial hair.

Oddly this is all happening in the same universe where it's apparently possible for one person to have more than one face — like how Rhodes used to look like Terence Howard but is now Don Cheadle, or Howard Stark is Dominic Cooper or John Slattery depending on the year, or Bruce Banner magically transformed from Ed Norton into Mark Ruffalo.

But that's a rabbit hole for a different time.