Foley's biggest mistake has been and continues to be thinking that it was all about him and the UAA and that UF would win regardless of the coach, facilities, ect because he and the UAA were the best around. This is in a great measure due to not understanding UF football, it is strengths and history.
UF football was defined by Ray Graves and Steve Spurrier as an exciting brand of offensive football with a stellar passing offense. UF football is also a product of the talent advantages, athletes who can run, in the state of Florida.
Due to his lack of understanding and apparent antipathy to all things Spurrier, Foley replaced Spurrier with DC with no vision of the UF football. When that did not work out Foley, probably with no real say in the hiring, made the $50 to $100 million mistake of taking 6 years of Meyer over another say 15 years of Spurrier. I can remember the smug around UF message boards saying of course Foley was so smart then because Meyer would be coaching at UF long after Spurrier retired. (Maybe if Foley understood, UF, its fans and its strengths in football, he could have lobbied Machen into the smart move.)
When Meyer flamed out, it was back to a DC again, even though Mullen was available then. Then when Muschamp failed, it was on to McElwain with Foley whispering in his ear that he should try to make UF into the UAL program and ignore the history of UF and its strengths. When that failed it was back to a spread offensive coach again ignoring the strengths of UF football.
This cycle of DC, spread coach, DC, UAL clone, spread coach has meant that for the largest part of the time since Spurrier left in January 2002, UF has had a mismatch between the types of players on the roster and what the coach wants to do. This mismatch is due to an AD not understanding UF, UF's football talent and the advantages of football in the state of Florida.
Ultimately this all stems from an AD thinking UF's success in football from 1990 to 2001 was due to the AD and his UAA staff. Thus thinking this was Foley's biggest mistake.