This bug is 100% reproducable, every time. Go to the shrouded clan hideout, and on the big elevator platform that brings you underground, jump as it starts to move downward. As soon as you land, gravity will just not apply to you, and you will be stuck in the air until you try walking in any direction, at which point you will fall to your death.

I suspect that the physics code is probably checking to see if you landed, and assuming that once you have, that the ground isn't, and therefore it doesn't even attempt applying gravity because it's assumed to not be needed. Such is typically true in 99% of cases, but still, this shouldn't be happening on vertically moving surfaces.