Have made two 'biggies' -
1) A few hours after passing PPL, I had taken off from 09 (the more unusual in the UK), then landed on 27 (RWY in use was still 10) about an hour later, even making the call "Final 09"! The disturbing thing for me was it was a delayed understanding - nobody else at the airfield noticed it, and it was only about 45 mins later in the car I realised...
2) Whilst working in my first commercial flying job, I succumbed to commercial pressure big time and elected to perform a completely non standard takeoff with a 20 kt X-Wind, uphill, because the trees at that end were not as tall (about 30 feet, compared to about 80 feet). The performance figures for nil wind and level grass and one stage of flap were only about 50 metres short of what I actually had UNTIL the trees. So I decided to go full power with no flap, then dropped about two stages a few seconds before the fence... It actually produced one of the most impressive climb rates I'd ever seen from that aircraft, but yes, what if the flap motor was burnt out, what if for some reason only one side had deployed, etc... We wouldn't have made it out if either of those had occurred.
First example was avoidable, but I learnt from it. Second example was inexcusable, and I definitely learnt from it...