Just fully trusting what was written there, I marked Joseph's first comment on this answer as a "great comment". Now Ulrike's comment told me that the first comment wasn't that great after all (no offense meant ;-). How can I get rid of this upvote?
It is not possible to undo comment votes at this time.
edit: this is now possible, but only once, and only within a limited time window after comment upvoting. Sorry, I thought the latter part was obvious, but maybe not :)