Most noticeable with shotguns. Sometimes when I shoot the firing animation will play (the shotgun kicks back ) but I don't hear the firing sound, don't see a shell being ejected from my gun, don't see any bullet markers on the map, and I don't see my ammo drop by one.
Likewise for nades (seen it with fragger but have heard from others about it happening with skyhammer) if I cook a nade and release it doesn't explode. I have a clip of it and I also noticed that when I first prime it, it doesn't play the fuse burning sound. When I then release it I can see the model of the nade in the world but don't see or hear it exploding afterward. In this case I do lose one nade.
What I've heard about skyhammer is similar in that they throw the nade, see the airdrop indicator, but no airdrop explosions. Also their cooldown goes to the min+ like you'd expect for a successful airdrop.