I've missed my share of flights; two of the more memorable:

Family vacation in the Colorado mountains, and we left in plenty of time to get to DEN 2 hours before departure. Except that there was a car fire on I-70 eastbound and traffic was at a halt for miles. We were among the many that UA accommodated with later flights.

Another time, I was on a business trip in Carson City, NV, and the negotiations went long, causing me to miss my UA flight to SFO for my redeye back east (something important the next day, as I usually avoid the redeye). So I took the next plane to the Bay area, which was to OAK. Friday night, last minute - no car service and no taxi would take me to SFO. I rented a car, drove across the bay to SFO, turned in the car and made the flight home, upgrade intact.