There is a joke going around about having finally an effective placement of a sponsor on a spoiler, which in normal conditions no one would see.
The truth about the Mercedes Benz CLRs taking off in 1999 at Le Mans (not once, but three times actually) is because they were specifically designed for that track, with the objective of having as less drag as possible, due to its very long straights.
In order to achieve their goal, the car was as long as possible but with a short wheel base to increase the size of the front and rear diffuser. Furthermore, they had their pitch angle almost at 0° (normally race cars have a negative angle for downforce).
All these factors meant that the car would rock its front and back at every bump or dirty air from a car in front, and when the pitch would reach 2.4°, it would just take off.