I've had my Ford Focus for over four years and done 60000 miles in it, and it's behaved perfectly except for one thing - the bloody reverse parking sensor packed up on me (one week after the warranty expired, of course...)

I did not ask for a reverse parking sensor nor did I want one. It just came with the car. Worse, I sometimes drive Mrs Steve's car, which does not have a reverse parking sensor, and I have to do a little chant to myself every time I drive it to remind myself that it will NOT bleep at me when I get perilously close to the car behind. But much worse than that is the unreliable reverse parking sensor. Of all the things that can turn unreliable on you, that's got to be one of the worst. It cost me £46 to get it fixed, over a year ago now, and I still don't trust the bloody thing.