This car was purchased new with the optional "active cruise control" (acc). under certain conditions the car accelerates unexpectedly. when the brakes are applied by the driver, the acc engagement should always be terminated. in certain situations the acc remains engaged despite the driver's use of the brakes. when the acc is engaged and detects an object ahead, the brakes may be applied automatically by the acc. if, while the brakes are still be applied by the acc and the driver also presses the brake pedal, the acc should be disengaged and remain off. however, if the object previously detected ahead has cleared and the driver releases the brakes, the acc control is restored and the car accelerates. this is a dangerous and confusing response. drivers instinctively expect any cruise control to disengage immediately and remain disengaged upon brake application.the bmw active cruise control does not perform this way in the situation described. this acc response is a repeatable defect that has caused me to experience a hazardous situation while driving.