When a player stands on top of a honey block being pushed or pulled by a piston, it moves the player with it. But if there's a lag spike at the same time, the block will move without moving the player, causing the player to fall off the edge of the block even though he should be stuck to it.

This is quite noticeable when using honey blocks as a "seat" in flying machines over more than a few blocks, where any lag can cause you to fall straight off the machine to your death.