Btw: Do I now have a reputation of -1 because my question sucked or simply because I asked 1 question and answered 0 so far (and 0 - 1 = -1)?

Your node has a reputation. Users get levels which are based on how many experience points they have (like in Dungeons & Dragons and several other role-playing games that followed its lead).

Don't beat yourself up too much -- to the person that wrote the code it looks correct so the bug gets glossed over. It is almost a prerequisite that a second pair of eyes is needed to catch this kind of bug.