You're right. Prime and BOC's overnight are two different things, but it's rarely seen Prime not following BOC' step.
It's rarely seen the two rates out of sync, but we did see it happened twice recently. Commercial banks stole 2x 25 base-points in the last few months (other wise some of your guys would be paying a 0.8~1% Mortgage rate). What I can predict is, if BOC's overnight is lift in future, banks should give those 50 BP back to customers on the Prime Rate.