Not sure if this has been mentioned but I've seen people get into trouble and never touch the throttle. The plane goes into a death spiral or gets into some weird attitude and the throttle is still going full while they are tying to pull out - the full throttle just makes it worse.

I've also forgotten to thoroughly check control surfaces - bought a new radio, forgot to reverse the ailerons.

Also - once I was starting up the airplane. It kept dying after a few seconds of running. I couldn't figure out why. Soon I figured out it was because I had not refueled. I went back to the pits, started talking and forgot to refuel. If the plane had not been totally empty it could have deadsticked a few feet off the ground in the climbout.