I sail a 1989 Nacra 5.5 uni.

The car came off in the middle of the track. Here is what happened, to be more specific:

1) A fellow sailor rigged the traveller car with a line that was a bit too thick.
2) The car snagged a rough spot on the track (possibly was angled in a funny way by the excessively thick line), and was pulled upward by the mainsheet.
3) I removed the mainsheet from the car, and pulled the car off the track.

My suspicion is that the car did not have enough ball bearings on the side that snagged.