I think it is if you are trying to post too fast after your last post.
It's good if that is the case, as it keeps spam-bots from attacking.
Although it can suck, if it halts legit posts, but then you just take a sip of your coke and try again
This is not the case.
I bet that theymos or whoever set the timeout as 0 seconds, and everyone is still seen as coming from the same IP.
It really needs fixing though.