50 times a second is hyperbole. Your heart cannot beat that fast because there is a refractory period between one contraction and the next. Before that could actually take place your heart would go into ventricular fibrillation, a disorganized electrical depolarization of heart muscle where many small electrical impulses cause chaotic and ineffective contractions of correspondingly tiny and disparate parts of the heart muscle.

I've heard this idea before, that you can die hours or days after an electric shock without realizing anything is wrong, but I think it is very rare.

An electrical injury can certainly trigger an abnormal heart rhythm, but you would probably notice most of those and, hopefully, go get a cardiogram.

If you felt normal, but had a serious electrical injury to the heart that would later lead to fibrillation, one would have to postulate some sort of abnormal re-entrant (sort-of circular) depolarization current in the heart muscle that was very regular, say 60 beats per minute, but that was prone to deteriorate into fibrillation over time.

That is what I recall about the matter, off the top of my head.