There is a story that Eric Owston (PCYC, Montreal) tells of the race where he was having simply terrible boatspeed problems. The boat was feeling like a real dog, and he was far behind the rest of the fleet. He did all the usual checks - weeds, sails, etc - and found nothing. But the boat sure was slow that day.
The truth surfaced when they got back to shore and had carried the boat to its cradle. On the ground nearby he saw the piece of carpet that he kept under the boat to protect the centreboard. It was soaking wet. A second piece of evidence was a few carpet fibres caught in the self bailers.
It would appear that when they had shut the bailers on shore prior to going sailing, they had inadvertently caught the carpet in them, and had dragged it all the way around the race course. Not fast! It had only fallen off when they were done sailing and were carrying the boat back to its cradle.
Add one more item to the pre-sail checklist!