Every time KITT drives into the mobile headquarters we can clearly see that the car just barely fits, with only a few inches clearance from each side, yet Michael is able to swing the door open and get out.
In many episodes, David Hasselhoff's stunt double looks absolutely nothing like him and is on screen long enough to be obvious.
K.I.T.T. is supposed to have an air-plane style steering wheel. However, in many episodes, the top portion of the stunt-car's normal, round steering wheel is visible through car's windows.
The copyright date at the end of the credits on the first (pilot) episode shows the roman numerals MCMLXXVII (1977) instead of MCMLXXXII (1982). Some people have suggested that this may have been when the idea was conceived rather than produced.
In almost every episode the landscape seen through the car-windows when Michael is talking to K.I.T.T. while cruising doesn't match the exterior views of K.I.T.T. driving along.
In many episodes, the cars used in crashes or in scenes where a car is falling in a lake are clearly model-cars "driving" through miniature landscapes.
In 'Soul Survivor' Michael is driving along the road talking with k.i.t.t on the opposite seat. When he stops the car after running out of petrol, he opens the door briefly and you see k.i.t.t isn't there. He is there again when he offers the car to the guy he flagged down on the road.
In "Knight of the Phoenix", a stunt driver wearing a hood is visible in KITT when the car inches for ward behind the guard as Michael is trying to escape Comtron. In the episode "Big Iron", when Michael and KITT are about to be buried under tons of gravel, some shots show that the tractor tumbles a Pontiac Trans Am, rather than the concept car used in the series.
The "tires screeching on pavement" sound effect is often added while KITT is peeling out on sand or rounding a corner on sand/gravel/grass. Screeching sounds would certainly not be heard while driving on such surfaces.
During the entire series, "turbo boost" is used both for high-speed chases as well as for jumping.