(For the non-US members, MacGyver is a TV- action guy who can use things around him or in his pockets to fix the impossible )
I had my moment this weekend. I set-off hoping to perform a metric century (62.5miles). I got 24 miles from home and heard a 'snap' form the rear of the bike. I look down and my chain is dragging along the road. I pull over and soon realize that the rear derailleur has lost tension on the chain as the internal spring has snapped. Im 24 miles from home, its hot and Im in the middle of nowhere. I flipped the bike over and manually set the chain to big ring on the front and big cog on the rear. Still the chain is too loose. I then pull out my trusty multi-tool (thank heavens I had it with me) and start adjusting the tension screw. By screwing it in all the way the chain looked almost normal. I then tried a few gears and the only gear the chain gave adequate road clearance was in 1st and "possibly" second. Shifting to a smaller cog allowed the chain to droop towards the road - not safe.
So I rode the 24 miles home in first gear at around 8-9mph. My legs had to spin like crazy just to keep the 9 mph average. I think that was the longest 24 miles of my life. I finished up riding 48 miles but it felt a LOT further