The sensor can be damaged by brake fluid, which can happen either when it's ejected from the reservoir during hard driving (if it was overfilled) or if some was spilled during a service. Unfortunately, this brake fluid-sensitive sensor is located directly under the brake fluid reservoir....
Happened to me too once. Now I just ask them to be extra careful about spillage and fill level during brake fluid service.
Likely selling end of 2015:
11 M3 E92 Le Mans | Black Nov w/ Alum | 6MT | Premium 2 | Convenience | 19" | EDC | Prem Audio | Moonroof | BMW Apps | Yoko AD08 R | Vorshlag Adjustable Camber Plates | StopTech ST-60 380mm BBK | Castrol SRF Fluid | ModMyNav.com rear camera | ECS Tuning Sunglasses Holder