Halfway through a 100km MTB event I got a second flat and found that my 'spare spare' tube was for a 24" rim.
That'll be for one of the kids' bikes then.... Bugger! Bugger! Bugger! Bugger!
I stretched it round the rim (sitting it inside the tyre was impossible) and with a little air in it just about got in to stay there without ripping or pinching.
Grit and gravel were everywhere and I was the saddest little bunny in the universe as fellow backmarkers whipped past me.
To my compete amazement, it got me to the end of the event. It stayed in that rim for the next couple of years.
I'm not saying there is a God, because I don't know. I don't believe in him/her anyway.
But... if there is a God, He or She looks kindly upon stupid MTB riders who fail to prepare.
Here endeth the lesson.