User blog comment:UsoppThePotato/An idea for critical strikes./@comment-11623843-20131003022542/@comment-5791947-20131003071017

League's critical strikes are pseudo-random, I can attest to that, as there are some champions who have very obvious critical strike animations (e.g. Tryndamere, Blackfrost Anivia, Aatrox), in which when you interrupt their critical strikes, the crit animation will still resume until you land a hit.

As I use 1% crit runes on everything, the moment I see a crit animation if I attempt to land it on a minion, I interrupt it and use it on an enemy champion instead.

True random will not work this way, bear in mind.