It's a good lesson to always take the Rockies seriously. I know lots of people who hit the trails with nothing more than a water bottle. But three years ago my son and I were doing some late-July elk scouting near Kenosha Pass. Temps in the parking lot were high-70's despite being at 10,000' already. Lots of folks were hiking in shorts and T-shirts. We go up to around 12,000, nothing extreme (for the area) by 1pm or so and over just 30 minutes watched everything change. A huge storm moved in, temps dropped to 35 and there was lightning, rain, and hail. That's when I discovered that I had stupidly left my own rain jacket in the truck - I had taken it out while repacking our lunches and not put it back in. But I always carry emergency gear and had a backup disposable poncho.
We left the mountain in a hurry, staying just inside the tree-line as much as possible (it's the devil's choice because with no cover, YOU can get struck, but trees can kill you when THEY get struck). The whole way down I was thinking to myself "this is how the article gets written. Experienced hiker killed by lightning in summer hail-storm in the Rockies."
We kept our heads and had our gear, and made it back. But it's a great lesson. The weather changes fast around here. Bring your gear, and double check it.