The best roadside fixes I saw were on a cycling training camp holiday on the Costa Blanca (Southern coast of Spain). It was the first day of the holiday and I was in a group of 20-odd riders about 25 miles from our hotel. Suddenly, from behind me, came the sound of catastrophic bike damage immediately followed by angry cursing in a strong Scottish accent. We stopped to investigate ...
It turned out that a young Scottish rider's budget Bianchi must have had a bit of knock on the flight over to Spain. The rear derailleur hanger had got bent so when he'd changed down into his lowest gear for a climb, the rear mech. had gone into the spokes of his back wheel.
The resultant damage was: rear mech. ripped off, gear hanger snapped, chain mangled, 2 or 3 spokes broken and the wheel badly buckled!
We pooled tools and know-how. First we removed the derailleur (it got a free ride back to the hotel in the Scot's jersey pocket). We then took the buckled wheel off, removed the broken spokes, whacked it against the road surface to take out the worst of the buckle and then re-trued it as well as we could. We couldn't get it absolutely true though, so we slackened the back brake off to stop it rubbing. We chose a compromise gear ratio (low enough to get up most of the climbs back to the hotel, but high enough not to spin out too much) and converted his Bianchi to a singlespeed bike!
The Scot limped back to the hotel, all the while muttering and mumbling about how his holiday was ruined, how angry he was, how was he going to get his bike fixed in time to ride with us again ...
But next day - a miracle happened! Scottish ex-top-pro Robert Millar was staying at the same hotel as us and he had a fleet of top-end bikes to test-ride and review for ProCycling magazine. It just so happened that the bikes were all the same size as our rider's wrecked Bianchi and Robert Millar lent him a different bike each day of the holiday in return for comments and opinions for the bike group review.
He got to ride a top-end Bianchi, a Pinarello, a Colnago, a Time ... (I forget the rest!)
How lucky was that! 