I used to ride a goldwing.The day before a weekend ride I was stopping at a store for groceries and forgot to put my foot down when I stopped. I caught it before the bike and I fell over.
On the first day of the weekend ride my fiend @Heavy stopped at a stop sign and also forgot to put his foot down. (I guess I was contagious.) He wasn't able to catch it and broke off the Left footpeg on his old DL1000. (He is sure to deny that is what happened but I saw it.. that's what happened.)
I did this once on my brother's vintage Moto Guzzi. Took it on a longish day trip with wife on the back. Stopped to eat lunch and gas up, so I turned off the stopcock. A couple of miles down the road: putt...putt...patooey. What the heck? Found my way to a phone booth (this was back before cell phones), rang my brother and promptly lost whatever cred I had as a motorcyclist. Thank goodness for fuel injection, I say.I saw a cartoon graphic of a women crying. The caption was, "I can't believe I forgot to have children."
I forgot about the gas tank reserve lever once after an extended time off the bike. Sitting at the side of the road trying to figure out why the bike stopped. DOH!
Forgot to put my helmet on. I was wearing a hat and noticed my bike was louder than usual. Drove by a street light, saw my shadow and realized my helmet does not have a brim.