In my experience, whenever a character falls because of a pit trap or a botched climb check there is always at least one player that that tries to catch them. I can't seem to find any specific rulings to this.
I'm not the sort of GM that likes to say, 'no, you can't do that' so I at least let an adjacent player make a reflex save if they ask (using the same DC as the trap save seems like a good idea).
Then perhaps a Strength check to pull them up (or avoid being pulled in themselves).
Does anyone have an official rule to this, or at least a solid house rule that can be used consistently?
Edit I've done a bit of snooping around and found SRD rules for catching character whilst climbing, which covers part of my question.
Catching a Falling Character While Climbing
If someone climbing above you or adjacent to you falls, you can attempt to catch the falling character if he or she is within your reach. Doing so requires a successful melee touch attack against the falling character (though he or she can voluntarily forego any Dexterity bonus to AC if desired). If you hit, you must immediately attempt a Climb check (DC=wall’s DC + 10). Success indicates that you catch the falling character, but his or her total weight, including equipment, cannot exceed your heavy load limit or you automatically fall. If you fail your Climb check by 4 or less, you fail to stop the character’s fall but don’t lose your grip on the wall. If you fail by 5 or more, you fail to stop the character’s fall and begin falling as well.
It would make sense that catching a character who is falling into a pit would use a similar mechanic, though probably replacing the climb check with something else.