Well, this could have two reasons.
One of them is a wireless keyboard or mouse. I'll explain it in a very simplyfied version:
A wireless keyboard has three sorts of signals. A signal that is sent when they key is tapped shortly, a signal when a key is pressed and a signal when a key is loosened. This is done to save battery power, especially when holding a key, the wireless keyboard or mouse only has to send two signals (keydown when you start pressing the key and keyup when you loosen it).
Sometimes, the last signal - keyup - isn't recieved, so the reciever still thinks you're holding down the button. You can easily solve this by sending another signal (read: press the button) again.
The second cause is a Soldat bug - which doesn't need a wireless keyboard or mouse. This usually happens at the beginning of a round. Again, tap the specific button to solve the issue. This is extremely annoying in realistic matches, as echo_trail mentioned in another topic.
Grtz, DePhille