During preflight checks on one aircraft, my instructor and I had to change aircraft last minute, I can't remember why. I did the shutdown checks and we jumped out, only to find a flat battery on the new aircraft so we piled back into the original one. I was feeling a little pressured by the instructor to get us going and he 'helped' by speeding up the departure checks.
The engine ran for maybe 2 minutes before I realised I had turned off the fuel without turning it back on. When I restart my PPL training I'm going to follow my checklist and try not to allow myself to feel as pressured, or at least recognise it and try to compensate if I am.
On my second ever set of solo circuits at Biggin, trundling down towards 21 a couple of pheasants were on the taxiway. They duly moved off as I approached, and I went over the upcoming circuits in my head. Clear takeoff, full power, 40...... 50 knots....... 2 pheasants on the centerline!
I luckily had airspeed enough to clear them without giving them a haircut although chastised myself for not twigging that they were headed for the runway.
My third (so far!) and most stupid was during PFL's with my (poor!) instructor. During my 'touch' drills I ACTUALLY changed tanks, without the fuel pump, engine idling, at less than 1000ft.... said instructor was pretty gracious about it.... even when the engine coughed a little in the climb out....