if the button is on the brake lever, the consequence of failing to reset it is merely that you have to reach a little further for the brake lever next time you go to use the brakes.
Campag brake levers need to be manually reset (so that you can run with the brake at increased reach/clearance if needs be) but some other levers (such as a few older shimano models that have a brake lever-mounted QR) have an auto-reset feature; the first time the brake is used, the button pops back out and everything returns to normal.
Caliper mounted QRs vary; mostly they don't open so far that, with a well adjusted brake, you have no brakes at all if you leave the QR open. However if you have badly adjusted brakes to start with, leaving the QR open will leave you with no brakes.
BTW most caliper mounted QR mechanisms have only two positions, but a few have an internal detent system so that you can run with the QR in any position safely; this allows 'on the fly' brake adjustments.
cheers