1 engine failure due to carb icing, in an RF4 with no carb heat control.... That came just a few minutes after a near head-on collision as I was descending at low power (and making ice), and a flexwing microlight was climbing steeply towards me. I saw him at the very last moment, and I'm sure he never saw me at all.
Had another very near miss once with a black PA28.
Was a passenger in a Jodel that crashed on landing. I've been a big fan of 4 point harnesses ever since!
In the "totally stupid mistakes that I can't possibly blame on anyone else" category, I too have done the "mixture heat" mistake - fortunately on the ground while being checked out on a Beagle Pup. And the one that could have been nasty was very soon after getting my PPL. I'd been following a written checklist, and must have got distracted 'cos I didn't check the canopy was closed. Fortunately I noticed just as I was lining up to take-off. I was in a Slingsby with a single piece rear-hinged canopy, and a happy outcome would have been most unlikely. I use memory checks now instead of relying on a written checklist, where it's too easy to miss a line by mistake.