A few times. A couple of the more notable: I flatted about 25 miles into a 60-mile out and back, and the effin' Conti Gatorskin tire was so tight I broke both of my tire levers trying to get it off the rim. Another time, about 65 miles into a 80-miler, I hadn't been keeping up on my eating and drinking, bonked so hard I couldn't see straight, stopped at a country store, got some food and Mtn Dew, and was still pretty well incapacitated. Both times made me thankful that God made cell phones.
'Course, if either had been a "real" ride (where I'd paid an entry fee and had some stake in finishing), I'd have sucked it up and soldiered on. ;-)
SP
Bend, OR