Currently it's possible to cast 2 downvotes on a post. You can flag as spam, thus casting a free downvote where you don't lose the 1 rep, and then actually downvote. It might be that you can't cast an actual downvote after the graceperiod on the vote from the flag has expired. I just flagged a SF question as spam and it allowed me to also downvote it right away. I did remove the downvote so as not to double dip and now I can't go in and downvote it again.
Or is the double downvote by design? I hope it's just a bug, given the change in weight of downvotes sometime in the next 6-8 weeks.