If you attack a Killer Bunny and it survives the hit, it becomes passive for several seconds afterward, hopping around like a regular bunny that's just been hit. It takes 5-15 seconds for it to become hostile again.
Steps to recreate:
1: Enter the following command (in Survival/Adventure mode):
/summon Rabbit ~ ~ ~

2: Punch it once, or hit it weakly with a weapon, so it survives the hit.
3: Watch as it suddenly won't hurt you for a very long time.