Occupied squares

[RobbyPants]

Lets say two people are in melee with each other. Then, a third person goes to move through his ally's square, provokes an AoO as he leaves it, and then gets tripped.

This should leave him prone in an already occupied square, if I understand correctly. Are there any consequences for this? No one gets shunted off a square, do they?

[NunoM]

A medium character can share a square with another, as long as one of them is prone. The prone character can't do anything else but leave the square. I saw somewhere that the prone character could be able to do some other actions, although limited, but i don't think that's an official ruling.

I didn't see a direct reference to this particular situation, but you can infer this in 2 ways:
- The ruling for the "Overrun" special attack (taken from here - pls let me know if this isn't an accepted source - also available in the "Rules Compendium"):

...
If you fail but aren’t knocked prone, you have to move 5 feet back the way you came, ending your movement there. If that square is occupied, you fall prone in that square.

Note: this is similar to the "Bull Rush" action as well
- There's also a special mention to "Sharing Spaces" in the Rules Compendium p. 62, for helpless characters:

SHARING SPACES
You can freely enter a square that contains a helpless creature, unless the DM rules otherwise, sharing that creature’s space. If you’re in a space with a helpless Small or larger creature that ceases being helpless, and for whatever reason that creature can’t share spaces with you, the creature must be or remain prone, attempt to grapple you, or leave your space at its first opportunity.

[RobbyPants]

Thanks!

That's sort of how I ended up ruling it, anyway. It turned out not to matter, since the combat was over before the prone character's next turn came up, anyway.