Once I was on an Outward Bound course learning some mountaineering skills. We were up on a glacier in the North Cascades, on a steep slope. It was summer and there was melt water running under the snow. I stepped in the wrong spot and the snow gave way beneath me. Lucking I managed to catch myself on some snow at the lip with an arm and a leg, and the instructor pulled me out. Had I fallen in I would have most likely gone deep under the snow in this freezing stream; definitely pretty terrifying.
The second experience I had, a few years at the end of high school, I went on a canoe trip in Maine that involved being flown into a remote lake in float planes. Once everyone else had been flown in, I flew in with just the pilot and a bunch of gear. As we were flying the pilot saw there was some bad weather where we were heading, and we were low on fuel, so he decided to land in a nearby lake to wait it out. We landed fine, then turned around and headed for a dock at a remote cabin. Luckily, the moment we landed the pilot opened the window next to me as apparently is standard procedure.
As we taxied back to the dock, right as we came within 15-20 feet of shore, a gust of wind caught the wing on my side and blew the plane up on edge. Slowly the plane tipped over.. I remember thinking, 'this should not be happening', and hearing the pilot radioing "we're upside down", then the window went black, the engine died, and we both scrambled to get out. I flipped around easily enough, looked up and saw the window with water pouring in. I had never been in a float plane before and didn't know how the doors worked, anyways I just instinctively went for the window.
I got my head out, and one arm, but the window was small and my other shoulder was stuck, so I was struggling and trying to wrench myself out, basically trying to survive however possible, if I break my shoulder so be it. All this time the plane is sinking, and the water is washing higher and higher, gradually coming to my mouth. I remember looking out at this lake realizing that I would die here; it was right after I graduated high school, when everyone say's 'now your life's beginning!' and this would be the end. Pretty intense, but you never really know when your time will be.
Anyways eventually I realized I was not getting out as it was, so I went back in then came out with both arms first, grabbed a strut and got through the window. Standing on the wind was SO exhilarating, "I survived!". Luckily the pilot made it out a minute later, and we were both pretty much fine aside from being wet and having had a hell of an experience.