In detective/ Christie-style crime films, whenever someone wants to knock out with a blow to the head the protagonist or an innocent party who has unexpectedly shown up is and about to see them at a place they shouldn’t have been, they misjudge the force of the strike and cause permanent brain damage or death instead of the intended 30-minute period of lights out.
Conversely, when the antagonist wishes to kill the inconvenient witness or murder their victim with just a single blow to the head, leaving them laying on the floor seemingly dead and bleeding profusely from the head, they find out in the morning that the victim had just suffered a nasty concussion and cut to their scalp, had regained consciousness an hour or two later, and were resting in hospital and expected to make a full recovery.