I'm driving to work down a busy city street and there's a fire truck with horns blazing coming in the other direction. As expected, everyone pulls over to the side of the road to clear a path. After the truck passes, a guy in front of me uses this opportunity to pass a few cars in front of him.
This is a foul.
You are not supposed to pass in this situation. An emergency vehicle is like the yellow caution flag in NASCAR, you are not allowed to pass. You slow down, wait for the hazard to clear, then you resume in the same order. You are not allowed to benefit when someone is in peril.