You are unable to interact with marines who are lying down. Unknown if this extends to xenos. If you mouse over them the game still thinks you're looking at the floor, and they do not show up under a right click menu. The only way around this is to alt-click the tile and interact with the downed marine from there.

Steps to reproduce:
1. Lie down (as a marine)
2. Mouse over your sprite and notice the game thinks you're mousing over the floor.
3. Right click your tile and notice the game thinks you are absent from the tile.
I am using the latest beta version of the byond client. I did not encounter this bug until I updated.