1. Stabbing Holtz in the back to set up his own hire as HC.
2. Not tabbing Meyer to be his OC after the inevitable firing of Coletto (who never should've been hired, obviously).
3. Advising Meyer against taking the ND job in December 2004.
I actually think 3 might've been more damaging than 2. Had Meyer been the OC, ND probably would've fared better in 1999-2001 than it did. But then Davie likely wouldn't have been fired in December. And by that time, I'm sure Meyer would've been poached by another school -- maybe BGSU in 2000, or maybe someone else that year or the next.
Especially in light of 2, it's disappointing that Meyer listened to Davie in 2004. Then again, Meyer saw first hand how dysfunctional ND's PTB were. And I suppose ND would've found a way to screw the pooch on hiring Meyer even without Davie's treachery.