Ever seen this post? Read it carefully, if you didn't. Mysticial's explanation is great.
To summarize the problem:
You register, start with 1 rep.
You posted an answer that was up voted 10 times, so you are now at 101 rep.
You offer 100 rep bounty on some question.
You are now at 1 rep (but you have 10 times up voted answer).
You delete your +10 answer.
But you still at 1 rep, system understand that you couldn't offer a bounty if you hadn't rep to offer it. So it gives you rep change and you rep is 1 now (otherwise it would be -99).
Now you undelete your answer and have back 101 rep, as if you hadn't offer any bounty - you get rep back for undeleted answer, but rep change is still there, but it should be reversed so as you are back at 1 rep.
As you see, that is a small problem. If I'll undelete back my post, rep change that occurred doesn't get reversed. So this is feature-request before I get suspended (however it is a very important bug). Remove rep change events if post is undeleted.