Check this out:

https://e621.net/post_versions?commit=Search&page=2&search%5Btags_added%5D=undefined

Starting a couple of months ago, a series of users have, apparently independently, gone nuts and added the tag undefined to a bunch of random posts each. The latest to be gripped by this temporary mania is the otherwise respectable tagger TheVileOne, who subsequently removed all of them again.

Is this some kind of error with an automated tagging helper? I've never used such things so I don't know what could be going on. Plus in some users' cases, a few dozen undefined tag additions are all they have ever contributed, which makes it seem less likely to be an affliction of power users.

There are several causes.

As TheVileOne discovered, one reason is that tag scripts are currently bugged out on the post changes page.
Another cause that I've seen before comes from an unofficial e621 browsing app that erroneously (and likely accidentally) adds undefined to all posts that the user views.