My worst tight caving experience was when cave diving in Gods Bridge in Chapel-le-Dale. I was maybe 35 in, where the cave is a very low bedding plane, requiring you to zig-zag about.
To cut a long story short, my demand valve completely failed in the very tightest bit, right at the limit of exploration. I checked the tank pressure and the tap, pressed the purge button, but there was still no air. I had my reserve in my hand (an essential precaution in a very tight spot), but I couldn't turn my head around. I tried to poke the reserve through under my chin, but I couldn't. Nor could I manage to release my helmet strap. Now I was a bit desperate, but I could see it got a bit bigger further ahead. So I struggled forward through the sqeeze until finally I could turn my head around and change valve. After a minute to recover myself, I made my way out very slowly and carefully!
I never went in there again, but it has been pushed a bit further since....
I think I was pretty lucky that day!
Mark