I was protested out of a race once upon a time when the (popped) rudder glanced off the ground tackle and the boats behind saw the mark bobble and "knew" what had happened. I watched. Cleared it by about three inches.
Any judge worth his salt will be very inquisitive of someone who witnesses something like that.
"How did you determine that he touched the mark?"
"I saw the mark move."
"Show me on the diagram your position, the position of the other boat and the mark."
[protestor draws diagram where it would be impossible for him to actually see the boat touch the mark]
"Did you see his boat touch the mark?"
"Well, no - but he must have because the mark moved!"
There are a lot of ways the mark can move without committing a foul.