I did twice, once by stupidity and once by circumstance.
The first time I was hiking with my son, just a day hike near Kenosha Pass in CO. When we left the truck at 10am it was about 55. I remember going through my pack because my son couldn't find his binoculars, and I thought I might have put them in there. Foreshadowing...
The first 3 hours were great, and we were down to T-shirts, it was around 75 most of the day. Then, over the course of about 30 minutes, a massive storm rolled in. The temp dropped to about 38, winds started gusting 30+mpg, and it was hailing like crazy. No problem, I went into my pack... and discovered that in the bag-dump that morning I had somehow not put my rain gear back in my bag!
Luckily he still had his, and I always carry one of those mylar emergency blankets, so I was able to cut a makeshift poncho out of it. Those things are miserable to wear and it was a pretty nasty hike out through the hail, but I remember thinking that if I hadn't had it, I could easily have been one of those "experienced hiker goes missing" stories you read about.
It was 75 again when we got back to the truck.
The second time I was extremely well prepared. I was with 3 other hunt buddies and we were sharing a 12-man teepee into which I had installed a crude stove jack and my buddy had brought a small box stove. We also had a Mr Buddy heater and plenty of 20lb propane cylinders plus all the gear you can imagine.
It wasn't enough. I mean, we survived, but with all that gear you would have thought it would be luxury. Except that was the year we got 18" of snow where we were, and temps were averaging -15 every night and 20 during the day. We ran that box stove AND the Mr Buddy heater flat out, and the thermometer we had in the teepee still only read about 35 most of the time. Crazy trip, one of our most successful, actually. But also the least comfortable.
Never take mountain weather lightly.