The worst was when my fiance and I went to Houston, Texas last November to get this old beat up mercury he got off ebay (he restores old cars). Within only 6 minutes(!) we were broken down on the side of the road, with tons and tons of steam and smoke pouring out the front and inside of the car. I guess the ebay seller just "forgot" to tell us about the problems in the radiator, and that we'd need to stop every 1-2 hours to refill the oil! OH, and "the stretching/grinding noise you'll hear is the brakes... their fine! just ignore them." - oh thanks for atleast telling us about THAT problem :p
Then we had the fun of finding out the windshield had been replaced by seller (who had absolutely no auto experience), it started raining and the inside of the car started flooding. Since the heat and everything else was broken, every window was fogging over (which out of everything was our biggest concern, since we could barely see a thing and wiping the stuff off didnt really help)
We certainly enjoyed our 13 hour drive back home in what I now call "the big red boat"! (I jinxed it apparently, now every single time we have to drive it, it rains!)