Let me re-phrase that then. You "shouldn't" deploy flaps in most airplanes at cruising speed or landing gear either for that matter. To do so on some aircraft will result in in-flight structural failures, also known as "stuff being torn loose and blowing in the wind and falling away". This typically precedes an accident report of some sort and/or an NTSB investigation. Some aircraft also have sensors that will not allow this stupidity without due diligence by the idiot flying, also known as an "override" switch. There are many pilots who thought they were smarter than the engineers who designed the planes. You can find their remains scattered all over the countryside. This is not to say engineers don't make mistakes, that is another issue