My biggest overall was shifting route on a road trip so we passed through a specific small town with a cafe I knew about - guy I'd worked with had left us to move home and was working there.
I probably added three to four hours of driving onto the trip.
It was a club trip at college, I only knew a few people in the club - they'd mainly recruited me because I could drive and could rent a car. I was the only driver for our car, was paying the fuel, and the rest of the car had no idea I detoured; we'd otherwise made great time and I pretended to be impressed and mystified when we rolled up and the other carload had been at the destination for an hour or two already. "You stopped for lunch? We stopped for lunch. How did you do get here so fast?!?"
The coffee was OK.
The trip was like a decade ago, and the one guy I'm still tight with from that group only worked our like a year or two ago that I'd hijacked that leg of the trip. He and his wife made a similar trip themselves - he suggested the same cafe "because it's on the way" - and then looked at a map and got really confused.