This is something I found across all S.E. communities.
Whenever I add a tag, and if I find out that the tag doesn't exist on the community yet, deleting this tag is a bit annoying.
See in the following screen capture what happens: when I select the tag to erase it, then press the ⌫ key on my computer, it doesn't delete the tag but instead keeps it and combines its text with the previously entered, validated tag without removing any characters from either of them.
So I have to forward delete at this point, and press Space to re-validate the previously entered tag.
I assume that BkSp/⌫ should delete what is selected, and not mess with the previous tag that was validated.
Mac OS X 10.10, Safari 8.0.8