This summer seems to have been brutal to cars. My most recent trip, 2 weeks ago, was just a quick drive over into West Virginia & back. About 8 hours into the trip, I noticed a vibrating noise when I was accelerating, and it was frequent enough that I decided to call my mechanic and see what he thought. This proved difficult, as the hills of WV aren't conducive to mobile coverage, so it took about 3 partial calls to him to determine that it was probably just a loose part that he could check out when I got back to Ohio. Since no lights on the dash came on and the gauges showed everything in the normal range, I kept going, but after another 6-7 hours it was getting more frequent and louder. I pulled off the side of the road and immediately the engine died as I coasted to a stop. Only then did the dashboard light up. Still without a signal, I saw a gentleman outside his house and asked him if I could use his landline call a tow truck, which, luckily, he allowed me to do. The driver arrived about 45 minutes later, and the first thing he checked was the oil, which was completely empty. I'd just had the car in for service a couple weeks before, and had never been told that it was leaking oil, so this was a bit of a surprise. I was still 4 hours from home, and there was nowhere nearby to rent a car, so I was stuck in WV for a night. My dealer sent a truck over to WV to get my car, and they're just starting to look at it, so I have no idea how much all of this is going to cost altogether, but the hotel, rental, 30-mile tow, and change of clothes I had to buy has already set me back close to $500. If there's a silver lining, it's that I got to spend the night in WV for the first time ever, which means I've stayed overnight in all 50 states now.
Other than this, the only unexpected delays/expenses I've had to deal with on roadtrips were a blowout on a gravel road in Montana, and hitting a deer at full speed (if it hadn't been a new car, it probably would have been totaled by the insurer). Only the deer caused me to pull the plug on a trip; the blowout, even though it led me to a tire shop that insisted I needed 4 new tires, which they needed to call in from another shop, only delayed me about 4-5 hours.