This is the series finale.
Marty did not have enough money for a haircut, but he does have enough money to get into the amusement park, which cost a lot more than the haircut.
Four years after this episode aired, David Kaufman would provide the voice for Jimmy Olsen (whom Marty claimed to be in this episode) on Superman: The Animated Series.
The clock on the clocktower is still operating in 1967. As every BTTF fan knows, lightning struck it on November 12, 1955 at 10:04 PM, stopping the clock for all time. (It's still stopped in 2015 in Back to the Future II).
Why doesn't 1967 Doc know who Marty is? They met in 1955 in all three films!
In this episode the word Brown-Out is used instead of the word Black-out.
Doc's purple shirt is white just as Doc yells at Biff for pouring soda over the generator, then is back to its normal purple afterwards.
The statue of the Pioneer has never been seen in Courthouse Square before this episode and is never seen afterwards!
At the time, in the US, the Secret Service investigated counterfeiting, not the FBI.
It is a bit unlikely that it would take 117 years (1875 to 1992) for invisible ink to become invisible, even if the deed was buried underground for that time.