No testing substitutes racing, especially when your goal is the 24 hours of Le Mans.
I remember Peugeot's first year and how the mechanics, after a few hours of racing had passed, had to frantically clean the sidepods in every pitstop the cars made, losiing a lot of time in the process.
It was a problem never detected in testing. Also remember the commentators saying that no permanent track used in testing emulates the conditions at Le Mans, because public roads are public roads, even if they're groomed up for racing purposes.
(I think it was dirt, asphalt, whatever, that got into the air vents at speed due to the particular areodinamics of the car, and got glued to the radiators, making the engines work much hotter than they should)