A couple of years ago I was doing shark tooth trips off Wilmington on a six pack boat. My friend, instructor, dive shop owner, cave diver, was always first to back roll off. One dive he started to go back, said "See ya", and we all shouted "Your mask!" as he hit the water with the mask on his forehead. After enjoying the rest of the 3 day dive trip horse-laughing him, a few months later a group found ourselves diving Catalina. After 3 days, we were on the last boat dive, a night dive. I was in a hurry to splash first. As soon as I hit the water, I realized I just wasn't going to get very far without fins. I swam and struggled over to the side as the DM frantically asked if my air was on, even though he had checked it. My no-mask friend just laughed and said, no, he just dives better with fins.
I could tell more stories about forgetting things. The hand piece to strap my Sartek canister light to my hand. (surgical tubing works.) One of the two twin batteries for my Sartek. (Found out to my delight it works just fine off of one, it just burns for 1/2 of the 8 hr burn time, and I typically don't do more than a 4 hr night dive anyway.)
I'd forget my rear if it didn't follow me around.