I created this community wiki answer on ethereum stack exchange beta some weeks ago. It has 12 upvotes and I do not get reputation for this. This is of course, by design.
However, yesteday I lost 10 reputation because someone voted up and unupvoted again. Here's a screenshot from my reputation tab:
As you can see, I did not recieve any reputation for the upvote (expected behaviour) but I lost 10 reputation for an immediate unupvote. I'm pretty sure this is a bug.
I am also wondering if this could be exploited to reduce other members reputation.