Not a horror story here (sorry, don't have one - haven't flown long enough, obviously
) but maybe a learning story will do.
Doing some slow stalls high up with the CFI one day, he says "okay, clean up and climb back up". I add full power, check the mixture, slowly drop the flap handle to gain speed and climb and the Cherokee is still climbing very slowly. I said "something ain't right". He asks "what do you mean"? "She is pretty anemic", I said. He commented that we're at 3,000', I tell him "she should still climb better than this". He asks "how do you know?". I tell him "I know, I've been flying her for months, this doesn't feel right". He just shakes his head.
So I start checking everything and find the Carb Heat handle in the On position. I pull it back up to Off (don't ask, it's a Piper thing) and she immediately livens up, climbing much better (considering the 6,000' DA). I ask my CFI "did you put the carb heat on?". "Yup", he answers. "Without telling me?" "Standard procedure". I said "only if required and it is not humid today, please don't touch my airplane".
Later, on the ground, I apologized because I realized I must have sounded impolite. He said it was alright and I was right to not wanting anything touched in my cockpit. I don't think the CFI did it on purpose to confuse me or test me. And even if he did, it is great training.
I was proud (as a student) that not only I could feel the problem but I could also troubleshoot it and find the cause.
Oh the good ol' days of student training, how I miss those. And my cool CFI.