Thought I'd post my stupidest and also the stupidest stack I've seen.
Was on my dh bike and came across this steep section I'd never tried before, so I thought I'd just check it out. Did that, then I sat on my bike which was on a few rocks which I presumed would hold me. Turns out brakes are there for a reason and I rolled over these rocks sitting down until I hit one which stopped the front wheel and yep, OTB and down the hill for me. Was by far the stupidest crash I've had and I wasn't even riding!
Saw this absolute tool of a rider coming down Tolosa on a voltage fr, trying to air every jump in one of his lowest gears. Anyway, he rounded the last two berms and was fangin' it for the last jump where off to the left is a 1m wide drainage channel, clearly visible and avoidable, he went staright into it at great speed and had the best stack I've ever seen! Front wheel dug in and the bike just flipped over and he went flying. Makes me laugh every time I think of it!