A spare gun. In all seriousness I have never needed anything but more magazines for my Glock. An extra trigger spring might be a good addition to your range bag.
I finally got my hands on a new striker assembly for my M&P. I've never needed one but I now have one in the bag. Trigger return spring is next, that should about cover it for M&Ps.
Fiber rods if you have a fiber front sight. Don't forget a lighter to install it. Those are easy to break whether you are running a plastic or real gun.
I carry a whole set of spare parts for my G34(everything but the frame and slide), but I've never needed it. The only thing I've ever actually replaced at a match is the trigger return spring.
When I shoot major matches or anyplace that I have to travel for more than 2 hours or so, Ill bring a spare gun.
If I were shooting a glock Id make a little parts baggie with a recoil guide rod assembly, a trigger return spring, a spare stock front sight, some fiber optic for my sight, and every small part for the top end, including a spare striker assembly.
A known, working trigger bar and connector assembly is nice to have. Since thats very close to what Robbie said, you might as well spend the extra $20-30 or so and get everything.
I dont like to watch from the sidelines, no matter the reason.