I've got -1 point from an answer I didn't write.
Is that a bug?
That's by design rather than a bug, under Help Center > Reputation & Moderation:
You lose reputation when:
your answer is voted down: −2
you vote down an answer: −1
So you get -1 and the recipient of the down vote gets -2. Presumably that's to attach a little pain to down voting an answer so they aren't used maliciously, although at the same time it does seem a little wrong to lose reputation when down voting a terribly wrong / bad answer that's only a whisker short of being eligible for deletion.
However if an answer is deleted, for example if it's really not an answer or is abusive that -1 will be refunded when the answer is deleted. That also applies if the person who posted the answer decides it's not really so good and deletes it themselves.
As per Pierre Vriens comment to expand on why I think this would only apply to answers if you are attempting to answer a question in good faith you are going out of your way to help the asker and future visitors to the site without any personal gain other than maybe a few votes. When asking a question you are requesting the help of others, and that might include expecting others to do work for you because you couldn't be bothered doing any basic research, or asking a question that is so poorly formed it wastes the time of the volunteers providing great answers to good questions even reading it. For them a few down votes is a good immediate sign they maybe should move on to a better question to answer.