ZDoom

Note: PIT_FloorDrop has no handling whatsoever of actors standing on other actors. So the monster on the floor moves down regularly, and the player is just falling down the next tic instead of being moved along.

Still happens.

[edit] Uploaded a demo with the problem.

[url=http://forum.zdoom.org/viewtopic.php?f=1&t=50968]Retest in 2.8.1.[/url]

Load holy hell v1.5 from [url=http://www.**LINK_DISABLED**.org/download?file=holyhellv1.5.wad]here[/url] on map05 with no monsters on, then warp to coordinates 2600,2075. Behind you you'll see a little pillar which can be activated by walking on the line.
Summon an archvile over the pillar (that's what is present with monsters on) then activate the pillar by jumping on the monster.

When the pillar goes down, you'll see this tiny jump effect..