Why does a car acclerate when on the break?

There should be a sensor at the base of the accelerator pedal, and another at the base of the brake pedal that both sensors sends the depression of the pedals to the computer, but, when one of those sensors goes, the ECM thinks the brake is the accelerator and both the brake and accelerator sensors work as one, causing the error. …you need to replace sensors, get em looked at, also could be a bad throttle position sensor, best to go to get the car put on a computer read out with a scantool at your local mechanic, which will tell you which sensor is gone and needs replacing, not recommended to drive the car this way, if it is the brake pedal sensor, it could cease to function entirely at random and accelerate at full revs when that happens. and only quick wits and a yank of the handbrake will stop the car from causing injury to you and others. ….DO NOT DRIVE THE CAR FOR LONG THIS WAY…!