Memorable, but not perhaps in a particularly good way;
When someone manages to turn the fuel "off" instead of "left" in a PA38, or more interestingly in a PA28.
The student moving the mixture to full lean/ICO, instead of reducing power with throttle or selecting the carb. heat on/hot.
Having your student rapidly move the controls fully forward, right to the stop on a stall recovery, totally ignoring the brief and demo.
My first EFATO, ignored all my own rules and turned-back.
My first airprox (airmiss as it was then), just going around from a PFL, to see a Tornado going straight over the top of us, followed by very choppy few seconds (but seemed like minutes) and a total inability to climb.
The first time I had an engine failure and seeing actually how much greater the rate of descent was with a windmilling prop, rather than with idle power.
On a lighter note and probably not really something to aspire to;
The first time you s**g one one of your students
