And schmitt-trigger inputs are inputs that have a hysterese, so that the output doesn't generate a large number of transitions when the input signal slowly changes from the valid region or a zero to the valid region for a one or reverse.

A standard logic gate expects the input signal to very quickly change state - something that is true when the wire distance is short, and you have few input gates connected to an output. The more capacitance you get on the signal, the slower the signal will toggle, and the more problems you will get on the input side. Another thing is that if there is strong ringing on the signal, the ringing may reach into the deadzone between a zero and a one, and may be detected as spurious transitions.