Electric handbrake tom-foolery

[Slyjoe]

Evening all, can anyone tell me why 9 times out of 10 the electric handbrake will release itself and let you drive off, (auto cars) and the remaining one time you go to drive off you sit there rocking up and down on the suspension, pulling against the park brake that has not released?

I've got a Mini and a Mercedes, both auto, both do the same to me without rhyme-or-reason.
Obviously, I've not read any manuals to identify the problem.