The server is running slow and your post times out. If you open a new window to go check and see if your post went through, it definitely will not have, and it is likely that someone will have already said what you were going to say. If you decide to chance it and hit submit again, you will invariably double - or even triple - post.
I was just about to post this, but you beat me to it, Cisco.
Ah yes. For some reason I get that a lot at work. Somehow the combination of firewall and response time makes things much worse than when at home. Sometimes you just have to take a deep breath and realize you won’t be the one to respond first.