An article in the WSJ mentions the fact that BMWs and Mercedes are programmed to electronically cut the throttle when "implausible" inputs are given, ie: enough throttle at the same time as enough braking force. If that is so, I imagine it only applies to auto transmissions, since there is no apparent limit in my manual Z4 to how much I can simultaneously press on the gas and brake pedals when downshifting by heel and toe technique.