When I go to delete a tag synonym (clicking on the ⊗ button, which I believe is a ♦-only tool), a popup asks me whether I want to “Really delete this synonym?” Since the tag names are in the far left of the window and the remove button is on the far right, it's pretty hard to be sure that I'm deleting the right synonym. This should really be mentioned in the confirmation popup.
Trying to reproduce this, I seem to have found another bug, maybe only in Opera (11.51):
On hovering the (x) buttons, they shift the color, but they don't shift back when moving the mouse somewhere else. This makes finding the right line even more difficult.
The tag synonym dashboard got a redesign a few months ago and this seems to have been solved. When I click 'Delete' it clearly shows which synonym we're talking about: