Just found this thread and without any intention of 'bumping' I'd like to share some of the mistakes I have made, hopefully someone can learn from it. I have learnt my home work a couple of times!
My biggest mistake was a cross country flight, not much more than 25 nm away so I was close to the field all the time. Took off quite late in the afternoon and the sun was going down but I thought I would be home before it was completely gone. However I wasn't and this was before I had NQ. The meeting with my CFI afterwards was nasty but I think I deserved it.
Minor mistakes, which easily occurs during training: landed with the feet on the brakes, forogt final check, realised on short short short final I wasn't cleared to land (went around, fortunately not the same flight as I forgot final check).