Referring to the 'stun' effect certain enemy attacks have and the procs such as life/mana drain on the mummies in AO:2.

The chance for these taking effect seems to be completely seperate from the actual attack of the character, while most attacks may miss, the stun/proc will land a lot of the time.

Life/Mana drain from mummies also stacks, while 1 effect per attacker seems fair enough to stack, a single attacker can stack multiples of the same effect, resulting in some very annoying mummies on occasion.