Originally reported at enwiki Teahouse.
Steps to replicate the issue (include links if applicable):
What happens?:
I see "The edit could not be undone because it does not exist or was deleted. " (undo-norev). However, none of the revisions involved are deleted. It's just a simple merge conflict.
What should have happened instead?:
I see "The edit could not be undone due to conflicting intermediate edits; if you wish to undo the change, it must be done manually." (enwik's version of undo-failure)
Other information (browser name/version, screenshots, etc.):
Revision 540bddfb1fb3ec recently did something with the error messages; perhaps it's related?