It might be a mistake. I haven't see the Rep GUI, but I wonder if it's real easy to do Rep- instead of Rep+. I have one Rep- where the comment was "Thanks". Usually the Rep- leave some kind of remark.
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Well from the other posts it seems like the OP knows why he got neg repped and it doesn't appear to be an accidental -rep. Just a difference of opinion in what the OP and -rep giver think is acceptable to say. Since rep is really just a popularity poll, the kind of post it turns out the rep was for can often result in some +rep or -rep depending on how people feel about it. Just a lesson for the OP to learn from for the future posts he makes.
I thought I'd explain something though about the part of you're comment I quoted, because I think a lot of regular members wonder about it.
So, can accidental neg repping happen? Yes it's possible.
To give any rep you just click the 'Give Rep' button. The only difference for neg rep is (if you can give it) you have to choose a radio button for agree or disagree before leaving the comment. So it's possible to accidentally click the 'disagree' radio button when you intend to give a +rep.
I accidentally did it once myself a long time ago. Fortunately it was before my rep power got too high, but felt really bad about it and sent the guy a pm telling him i was sorry. As soon as enough time had passed for the system to let me give him rep again, I gave him a +rep to give it back again, (then again a few months later so he could have the +rep I had intended.) That only happened once though, and since then I ALWAYS double check that the 'disagree' option doesn't accidentally get selected.
While that can definitely happen, it actually isn't a common problem. More often it's just that some people who think they've got a neg rep with a positive comment are really just seeing a neutral rep.
A +rep will add to your rep count and show in your usercp with a green icon next to it. A -rep will deduct from your rep count and show with a red icon next to it. But if a new member with low rep power gives a +rep, it doesn't affect your rep count either way and will show as a grey icon.