After Barth is thrown into the water by Michael, the top of his jacket progressively goes from wet to dry each time we cut back to him.
When the gardener sprays water over K.I.T.T., covering the hood in water, and K.I.T.T. says "Would you mind raining on someone else's parade?", the following close-up shot of K.I.T.T. when he says "Thank you" shows the hood is completely dry.
Goof when Barth is waving his gun at Michael, ordering him to get into his car. From Michael's point of view he is holding it in his right hand. When the shot changes to the reverse side, the gun is in his left hand.
When Michael gets into K.I.T.T. to drive away after offering to help Griffin, Griffin has left his trunk open, but in the next shot showing K.I.T.T. starting to maneuver away, the trunk on Griffin's car is closed.
The file on Griffin says he's a British citizen injured during World War II who "won a Purple Heart." First, only Americans are eligible to receive Purple Hearts. Second, they aren't "won" -- they are awarded.
The first scene with Michael and Grace talking to each other in the garden is full of inconsistencies and contradictions. For instance she says "This must be the beginning" but then goes on to say how she's spent her whole life terrified of the questions, so it's not the beginning at all. Then she says she didn't know her father as a notorious cat burglar but as a loving father; she then goes on to say she's the only person who knows his burgling techniques, which is in direct contradiction to her earlier statement.
When KITT self drives to follow Michael and Barth the driver can be seen behind the driver's seat covered by a cloth of the same color as the car's interior. Most likely looking through the space between the headrest and seat.
KITT asks Michael why rubies are valued so highly, and says "According to my data, a ruby is just a form of carbon." KITT seems to have rubies and diamonds confused. Diamonds are carbon crystals. Rubies are corundum - crystalline aluminum oxide - with trace quantities of chromium for color.