Name the cases when Fergie proved you wrong?

Ditching Ruud for Saha and putting all his faith in Ronaldo/Rooney. These lads were only 20 at the time.

Seems obvious now but I remember that league cup final against Wigan when he started Saha ahead of ruud and the football we played was superb. It was in some way Fergies fledglings 2.

Ruud was the best out and out striker Ferguson had at United ever in my view so that was a huge gamble. Be like selling shearer at his peak for a 20 year old.
It is what Real Madrid and Man City currently do. Get rid of star players just as they are at the top of their peak or have already, albeit unnoticeably to most, started to decline. Saf was the master of that until the Glazers came and forced him to keep players beyond their use by date.
 
That's exactly why we didn't bother. We wanted Modric but we didn't want to deal with Levy again.

The midfielder we wanted after Modric was Thiago who United scouted for about two years with Martin Ferguson attending Barca matches in person to watch him. Fergie got the deal done going into the 2013 summer as a welcoming present for the next manager, all that was needed was for Moyes to sign off on it but he turned him down.

I remain convinced that we were bringing in Thiago, Herrera and a left-back (possibly still Shaw) that summer, then Moyes saw the brief of "midfielder and left-back" and decided to scrap the plans we had and waste all summer on Fellaini and Baines, while seemingly remaining convinced himself that we were signing Ronaldo and/or Bale.

As for what the OP asked:

Replacing Beckham with Ronaldo.

"Replacing" Keane with Carrick.

Darren Fletcher's general involvement with the first team when he first broke through.

Ditching RvN for Saha/Rooney.

van der Sar.

Evra/Vidic.
 
Literally every week with Fergie. He'd put a team out where you'd go "he's lost the plot" and then Darron Gibson would dominate a semi final of a Champions League and you'd go "no, he clearly is a genius, and I'm just an idiot fan."