Currently, the system makes you think you can upvote/downvote your own question/answer but then you get notified that you can't. Why have the up/down vote buttons there then?
In general, if some operation is not allowed, then don't put UI for that up or make it disabled so it is apparently disabled - no post operation error panels please?
ps: hope this is not a dupe, but if so, just comment it and I will remove
(+a joke: a user notification panel message: I deleted this file. Is this a problem? Yes/No.)