The problem is that the r2-d2 tag itself does not exist. There is only a synonym pointing towards it, and trying to rewrite a synonym to a non-existent master tag simply doesn't work. At some point after that synonym was created, all the questions tagged with it got retagged to something else and the master tag was automatically deleted.
This particular case, though, causes something interesting to happen. Technically synonyms shouldn't exist pointing to tags that don't exist themselves. So rather than the system picking up on that, it's assuming you're trying to create a version tag because of the "2" on the end of it. All it's seeing is the "2" and the fact that it's being rejected for some reason it's not programmed to recognize and it's just going with that reason because the "conditions" match. (Sidenote: if it hadn't ended in a number, I believe the system would have created the tag again, which causes another whole problem in that the synonym system lets users without the create new tags privilege bypass it and create tags which are masters of a synonym.)
I've gone ahead and just deleted the synonym, which should cause everything to return to normal regarding that tag. If anything, we should probably just automatically cleanup tag synonyms which involve tags that don't exist anymore. This is already covered when a moderator merges a tag, which will automatically transfer all synonyms over to the new master tag during the merge process.