If you put a large amount of mobs in a small spot (Using something like trapdoors) then walk into them, you'll get thrown really far back.
This GIF shows the bug. The cage has 298 creepers in it.
http://giphy.com/gifs/3o85xCeZoTowTLCxQ4/fullscreen
MC-91441
Multiple mobs in one place creates giant knockback
Resolved
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The player is still taking input for knockback, even after they have touched the first mob.
If more input is disabled until the player is knocked back, this will be fixed.