People think I'm horrible and have bad hands because my horse constantly has sores on the corners of his mouth. I have tried almost every bit under the sun, rubber, copper, sweet iron, rolls, snaffles, gags, combinations, full cheeks, solid one pieces, dog bones, bit guards larger, smaller. All of them make the corners of his mouth peel and when he is feeling fresh he can get a little raw. (He pulls and tosses his head, I try to just maintain pressure) I have asked a few trainers/vets about it and I was told that my horse just has a perpetual babies mouth. He won't develop the callus's needed to help prevent this. I try to keep light hands while not letting him get away with his antics.
Sometimes it just happens for one way or another. It could have been a pinch, a quick rub, and irritation. If it doesn't keep happening I wouldn't worry too much. It happens, you have metal sitting between two pieces of soft skin, it moves around, gets pulled on and what not.