A week ago something on my home office network started using a heap of bandwidth. My average daily download went from 4GB/day to 28GB/day. By the time I noticed a few days later my monthly download was about 120GB higher than it usually is at this point of the billing cycle.
What happened? There is a bug/oversight in *coin code that causes a client to misbehave when a fork occurs; the client repeatedly asks for the same blocks from peers on another fork, but of course rejects them because they don't match the local copy of the chain. Client A doesn't slow down its rate of requests to Client B, even though it is getting back exactly the same data each time and will never resolve the fork, plus Client B does not ban Client A for abuse either. It's just two stupid computers stuck in an endless loop.
I think in this case it was either
grain or
mintcoin that caused the extra 120GB of download, but it can happen to pretty much any coin when it forks. I'm running over 60 altcoin clients so I guess I need to keep a better eye on things.
tl;dr if you have a download limit and leave old clients running you may end up in some trouble.
