Back when Dressrosa was releasing weekly almost everyone believed that Kaido is Meadows in disguise. It had to have been the dumbest theory I'd ever heard. Their reasoning was that Kaido wanted the Mera Mera no mi, but didn't want to reveal himself. I was like, the man is a Yonko, he could literally beat everyone on that island and take the fruit for himself, why the hell would he go undercover? Also Meadows looked nowhere near what you think a Yonko would look like.
Other dumb theories: Sorry but the "Zoro's closed eye has secret sharingan powers" theory needs to die.