I just killed a big magma cube with a knockback sword, which caused its "corpse" to fly off of a ledge and disappear mid-air.  When it spawned the smaller magma cubes, they spawned with no downwards velocity, which made them hover there and it looked kinda goofy.

I think technically, because conservation of momentum could exist here but doesn't, this is a bug.  Would love to see this tiny physics oversight fixed in a future update!