Very early in my engineering life I was checking a HS125 pitot head after a flight, unfortunately it had been left switched on. The burn was there for some time, but I never ever repeated that mistake!
Standing up under the TFE731 engines on a BAe125 and denting the top of my head on the witness drains was a regular occurrence, but again over time I learnt.
On the flying side, just like another poster, I'd opened the cowling on a PA28 when the wind blew it shut on my fingers - "ouch" was not what I said. Dumb.
M.E.