When I attack a mob and take damage from them my weapon (sword, bow, pork chop) disappears. This only happens when the weapon is in mid swing. In the case of the bow it happens when it is drawn.
To duplicate this:
Trap a skeleton with some blocks
Move 10 to 15 blocks away
Attack the air in front of you just as the arrow is
about to hit.
Alternate duplication:
Stand next to a hostile mob
Aim crosshairs so an attack will miss
Spam attacks or time your attack to coincide with the
mobs attack.