I believe "bobbyv" hit the nail on the head. "Knocking" is a no-no, over a prolonged amount of time of; city driving, up hills, high speed, and basically anything which stresses the engine from medium to hard, can be very bad.
The result, possibly over-heating of the combustion chamber (cooling system not designed for the added heat) and a warped head. Or possibly a cracked/broken piston under extremes (acceleration, heavy stress). Anyway you cut it, if you abuse the engine, it will fail.
Hope this answers your questions.