Last year in a 65 mile gravel race, the combination of high heat, high humidity, and poor nutrition (damn hotel was supposed to offer a full breakfast!) did me in. I started cramping at about 30 miles -- cramping so badly that I couldn't pedal up the very steep hills, but could barely climb off the bike to walk as my legs were both locked up. Had no energy, so every pedal stroke was torture. At some point I missed a turn (GPS wasn't working and the course markings were almost nonexistent), so ended up riding longer than the race distance but still ended up back at a mid-route aid station through sheer coincidence. Fortunately, a guy was heading back to the finish line and gave me a lift.
First DNF in 15 years of bike racing. It was the single worst day I've ever had on a bike.