It's often considered bad luck to wear a helmet that's been dropped. I don't know where this superstition came from, but it's one of the few that is absolutely true. Motorcycle helmets are designed to protect you against one big hit. When you drop a helmet, it can't tell whether the cat knocked it off the table or this hit is The Big One (TM). Either way, plastic can crack, foam can compress, and the helmet won't provide as much protection as it did before the hit. So do yourself a favor and be very careful not to drop your helmet.