I recently went on a 60 mile gravel bike trip with a rather large group (50 riders) in the PNW. It was ~May of last year, and a freak storm came out of nowhere over the mountain, while we were in a large national forest. We were all wearing shorts (weather was in the 60s), and woefully unprepared as the temperature dropped into the 20s. I had thin gloves with holes in them. The storm dumped sleet on us and we had to descend a mountain in the snow for an hour in those temperatures, soaking wet. After 30 minutes I couldn't feel anything past my forearm, or below my ankle, and we had to bike about 2 hours in those conditions. The group got split up, and we all got hypothermia. it's a miracle no one died. I somehow got a flat tire and my sealant didn't fix it. Apparently I turned to my friend and asked if my rear tire was low and he lied and said it looked fine, knowing that if I stopped to try and fix it, I wouldn't have the coordination to actually repair the tire with how cold my hands were, and I would have really struggled to get going.
Entire thing could have been avoided if we had gone into it with the assumption of "When you enter the mountains, assume that any cold/wet weather is possible". Bring hand warmers and/or safety blankets, or even just a big roll of aluminum foil. Someone in the group had brought an amazon package specifically for that reason, and they shoved it in their jersey to block wind and keep their core body temperature up. They were fine, the rest of us were not.
EDIT: another storm I'm just remembering - not mine, but my dads. When he was 18 he was hiking on the side of a ravine in California with two friends. They were supposedly trying to find some caves in the side of the hilltop, and my dad grabbed some loose rock and fell - over 100ft straight down, another 200 tumbling at an angle. He spent 3 months in the hospital, mending a fractured skull and jaw, 2 broken vertebrae, and a whole lot of other injuries.