This happens to me fairly often. Generally, it's just an error where you've entered a server and, for whatever reason, your connection timesout upon entry but the server's already loaded on your screen. You'll notice when you press the ESC button that all movement stops-- this is because now the connection is just on your end, much like when you play against bots.
That's my ASSUMPTION, anyway, to be truthful I'm more just talking out of my ass with what I think causes it.
Anyway, simple solution: ESC+Leave Server, and then just re-enter. If it persists, find another server. These things happen from time to time, just an annoying and unfortunate bug.
I might have a bug to cause when I play online if I shoot someone close up with any gun exept Barret and M79 I can fire every last bullet at them and they still wont die
That might actually just be lag, really. What's your average ping in relation to the people you're firing at?