Almost too many mistakes to name.
The single biggest failure in terms of player transactions was obviously (IMO) purchasing Amauri for 22.8M euros (Not including Nocerino) the summer after our legendary ADP - Trez partnership flat out dominated Calcio by scoring 41 league goals.
That money should have been spent upgrading our defense. The quickest way to succeed (in virtually all team sports - unless you're blessed with the good fortune of multiple, young, otherworldly offensive talents), almost always occurs by building from the back out; you simply need that foundation to help neutralize growing pains... which obviously helps explain the extent of our current problems. We're always vulnerable, leak goals like a fucking sieve, and it prevents other players from being able to focus properly on their own position.
It's a bit like if one injures their knee, and doesn't properly care for it. They wander around compensating for the pain by shifting their weight (balance), and low and behold, suddenly they've developed a fucking hip issue on top of the knee injury... and without serious attention, things can really spiral out of control from there.