Came home to find out my brake lights were stuck on. Apparently there's a little rubber disc that goes on the brake pedal arm that pushes the switch to shut off the brake lights when the pedal is up, and it disintegrated. Found the pieces of it on the floormat when I went to check the switch. Luckily I found a bolt laying around whose head was almost the exact thickness of the disc, so I just shoved that in the hole and now the brake lights work normally again.
There wasn't room to get a nut on the back side (the side of the bolt is right against metal), but the pedal has to be pushed way down to even get it in or out, and it's relatively snug in there, so I guess this will be the permanent fix.