ippiki_ookami said:

It also looks like you deleted a bunch of valid tags in the process. So watch what you're doing with the tags.

Actually that's a bug I reported a couple months ago but hasn't been fixed. If two people are editing the tags at the same time as each other, the site will automatically remove the first-person-to-save's tags when the second-person-to-save finishes their edit. The only tags it will "keep" are the ones both people told it to add by lucky coincidence. There's also no way of knowing whether it's doing this when you're editing because it does it invisibly behind the scenes. So unless you obsessively check the tag history every time you finish tagging a post, you'd never know if it removed a simultaneous person's edit on your behalf. Though it will credit one of these people for the site's automated removal of the other person's tags, even though they had no way of knowing the other tags were ever there in the first place since it happened at the same time. It's a serious bug. It happens most often when images are brand new and more people are likely to try and edit the tags at the same time.

You can actually test it and see for yourself by editing some new posts and checking the tag history of each one. Sooner or later you'll edit the tags at the exact same time as someone else, and (either you, or they) will be credited with seemingly removing every-single-one of the other person's tags and replacing them with your own. It's not intentional, it's due to this un-fixed bug.

There's another way you can test it as well: open the same image up into two different tabs/windows in your browser. Make one tag edit to one of them. Save it. Then switch to the second tab's copy of the page, and without refreshing the second tab's version of the image, make a different tag edit to the image in the second tab and save it. Then look at the tag history for that image. I guarantee you that (unless this bug has been fixed between writing this and when you try it), the tag history will list you adding the first tag change you made. And then in very next tag history entry it will list you as removing your own first tag edit and replacing it with your second tag edit. A removal you did not intentionally do, and it did for you silently behind the scenes. That is the bug in action.

Now, the site used to handle it differently and it didn't used to overwrite one edit when another was saved like it does now. It kept track of what was the most up-to-date version of the tags even if your version of the page had fallen out of date, and it didn't used to remove tags that weren't there when you edited it (this kept you from removing simultaneous tag edits). But now it picks one edit and deletes the other. I reported it in the bug report thread months ago but it seems to be overlooked because it's still unfixed. So if you want to give someone hell over it, talk to Tony or Spight. Because it needs fixing.