Did you fit them the wrong way round and wonder what the 4 little blocks of black rubber were as they flew out in front of you as you failed to slow down when you hit the brakes?
Nope, but it DID happen to me many years ago when I were wee lad. Being Mr. Smartarse, I'd been playing about with what had been my older brother's bike; a Rudge Clubman, y'know, taking things off, putting them back on again, see how they worked etc.. But on this occasion I'd been playing around with the brakes. Later on belting up to a set of lights which of course changed a precisely the wrong moment, hit the brakes and watched the two front blocks jettison out of the shoes and carry on ahead of me

, thus promptly halving my braking power. A salutory lesson in ensuring one always properly tightens up what one takes off when one puts it back on - !