When the ice cream melts down onto Biff's shirt and jacket at the furnace, it makes a dark, chocolate mess. When he runs back to the ice cream truck to get away from the guy shoveling coal, the mess has completely disappeared from his clothes.
Biff locates Bonnie by following her footsteps in the flour on the floor from the bedroom into the living room behind the sofa. But when he walked through the living room and the bedroom, he made no footprints on the floor. Being heavier than Bonnie, Biff should have made footprints, too.