Alan Ball heeding the advice of some bloke in the Main Stand with a transistor radio has to be up there.
Appointing John Benson always seems to get overlooked.
Peter Swales offering Joe Royle the job live on TV, which caused him to turn it down when the fans and staff at Oldham pulled on his heartstrings.
The abortion that was the Umbro Stand. Another missed opprtunity.
Bringing Big Mal back and allowing him to dismantle a very good team in an unbridled fashion.
However the biggest mistake has to be showing our not so noisy neighbours kindness and benevolence on more than one occasion in the past, to enable them to survive and flourish, for them only to forget this fact as soon as it suited them.