People are always asking about this one.
The silenced G3 knife glitch is simply a time delay, from when you knife your opponent to when it actually kills them. You'll only see that with that specific weapon in your hand. You can switch to a pistol and it will be fine, or pick up another weapon.
[youtube]http://www.youtube.com/watch?v=E4tq0ZJOrCw[/youtube]
It still kills, but it requires both attacker and defender to remain almost motionless.