We had a fascinating bug on EC2 -- we could connect to the instance, but no network traffic made it out. It wasn't security group problems, it was literally a really weird bug in EC2's network that we somehow triggered, the engineer over at Amazon that looked at it was really excited when he came across our case as it was so weird, heh. They fixed it, I can't remember exactly what was done on their end, but it was one of the weirder problems I've attempted to debug. Nothing I tried worked!