I set up my tent in what looked like a good spot. It rained that night and I woke up in a literal puddle. What's your biggest camping fail? The kind that seemed fine until it all went wrong.
I don't know about the worst as I had so many but I know the best when I was ordered to sleep in an armored command vehicle and I was all buttoned up nice and toasty warm and it snow like a sun-of-gun and got cold.