Proper etiquette is to find the guy in the pits after the incident, make sure he's ok, and apologize. If he tells you to get the fuck out of his sight, you get the fuck out of his sight. If he's not too crazy mad at you, you offer to drop what you're doing and help him fix his bike. This includes grabbing any parts that you have that might be needed to fix his bike. This is generally just what ive seen and my opinion. I have never seen anyone pay for, or offer to pay for, another guys damages. Its kind of just part of the risk we all take I guess. Luckily, I have yet to see a straight upfist fight because of something like this either... but I haven't been around THAT long and I'm sure it's happened at some point.