True. The one that comes readily to my mind is where the trio of apprentices try to prove out that a driver would lose 10lbs of weight driving a race car in the heat. They weighed themselves clothed in their racing suits both before and after the experiment. What they failed to take into consideration is how wet and full of sweat the driving suit may have been after the fact. They should have weighed (discretely, obviously) naked for the actual before and after weights.
Oh wow, that is so obvious. Ouch. Even if you dried the cloths out they probably weight more from the salt build up.