Can you push an enemy into another enemy's square?

[Hypersmurf]

Don't have my book handy, but I think I recall that if there is no legal square to move into, the target is either knocked prone, or gets a save to avoid being knocked prone.

No, the forced movement just fails.

Or rather - assuming Push 1 square - you push him into his ally's square (since he can enter it by walking, it's valid), but then since he can't end his movement in the ally's square, he ends his movement in the last legal square he occupied... which is the square he started in.

The distinction is important, since he may suffer effects of entering his ally's square (if it's got a Blade Barrier in it, for example!).

-Hyp.