What happens if you cast plane shift and then attempt to shift to the plane that you're on? I can think of a few reasons this might happen in-game:
For some reason, the PCs don't know what plane they're on, so they try to plane shift to their current plane.
PCs who don't have teleport memorized find themselves in over their heads, and want to use plane shift to escape from their current location (but don't actually want to go to another plane).
I can think of a few possible results:
The spell functions as normal. The targets immediately move elsewhere on the same plane, arriving 5 to 500 miles from their intended destination.
The spell fails without explanation, so PCs in situation #1 still don't find out what plane they're on.
The spell fails, but it is somehow obvious why. PCs in situation #1 find out that they're already on the plane they were trying to go to.
Are there any possibilities I'm missing? Has this issue been addressed explicitly in any edition of D&D or Pathfinder? What actually happens in Pathfinder?