One of my students attempted to add takeoff power in a Duchess with 35 degrees of flaps down. I just stopped his hand from adding any more power and said After-landing-check...please.
Various other students have forgotten: Doors, windows, x-ponders, mixtures, aux pumps, parking brakes, tie downs, chocks, retracting gear and/or flaps aft T/O...if it's on the checklist, I've seen it be forgotten.
I have managed to intentionally distract every student to ever fly an RG with me to forget about the putting the gear down for landing. I call for the go-around and it's usually never forgotten again.