I've had one or two over the last few years. I was walking along a flat gravelly and sandy path, on my way to Florac on the GR70, fence on my right hand side, raging rocky river on my left. Suddenly the footpath just crumbled away from under me. I have no idea to this day how I managed to grab a clump of brambles before I fell into the river, but I did and I managed to pull myself onto the path again. I was so scared and shocked I just lay there and could hardly move for some minutes! My hands were cut badly and they were painful for a couple of days but I survived!
Whilst walking towards Champex on the Tour du Mont Blanc I crossed over the top of the Fenetre d'Arpette and whilst descending over some large boulders, in an instant I slipped and luckily fell backwards, my rucksack cushioned my fall (I could have I think broken my back?) but I was only bruised and shocked. I just hit the deck and then fell and rolled to one side. If I had fallen forwards I'd be no more!
The first time I walked from Kirk Yetholm it was blistering hot and I ran out of water well before I got to Byrness! So scary, my throat was like a rasp, my thirst was overwhelming, a very scary period of time! Since then I've always made sure my water supply amount is more than good enough!
On odd occasions I've also had irrational thoughts as well, especially when crossing large boulder areas. Going up or down hasn't worried me particularly, its the horizontal crossings that starts me thinking, what if it all moved and I become part of an avalanche?