Why did my move not count?

I was playing a bullet game and I was down to like 0.5 seconds and I moved but it did not count and my screen said you move does not count! Why did this happen?

Too late

I'll ty

try

It's possible you lost connection and the move didn't reach the server in time. 

Could be any number of reasons.

You pre-moved and that move was illegal (it might have been legal before your opponent's move, but after it, it would put your king in check)

You did not pre-move and spent 1/2 a second, leading to your clock reading triple-zeroes.

You did not pre-move and you mouse slipped and while you may have intended something like Bg2-c6, you really went Bg2-d6 via a mouse slip - or any other neighboring square.

I think I remember it saying something about the server.

I was not using pre-moves and I moved and I was not using a mouse