I think this is what happened:
- I reached my cap
- Someone downvoted (so now -2 from cap)
- Someone else upvoted (only get +2 instead of +10, cap reached again)
- Downvote retracted (+2 refunded, now +2 above cap)
Everything's fine according to the audit, though (http://stackoverflow.com/reputation), so maybe this is a known bug that isn't worth fixing.
Note: by "fine" I mean that I do not actually have the +2 pts above cap according to the audit.