I watched GF3 again and I noticed that Michael imparted a lot of advice to Vincent but forgot one piece of advice his Father gave him.

Don Altobello got in Michaels car and suggested that there should be a meeting to calm things, get everyone together, which resulted in the helicoptor hit. Afterwards Michael was trying to figure out who the enemy was and only did so when in the throws of a diabetic stroke. He should have remembered the advice he got in his Fathers tomato garden many years earlier...

VITO CORLEONE
Now listen -- whoever comes to you with this Barzini meeting -- he's the traitor. Don't forget that.

That's true. Michael was getting old and starting to make this mistakes. Vito himself did it similar in the first movie. I doubt it that if he would be younger, Solozzo would hit him so easy.

Michael made other mistakes that his father never would have made. For example, in the penultimate boathouse scene in II, he gives the order that "I want it [Roth's airplane] met" before Rocco, Neri and Hagen. Vito always gave orders involving death and violence to only one person at a time, the better to prevent having a rat become a corroborating witness.

Michael's biggest shortcoming IMHO was the same as Santino's...
he could not control his temper. Obvciously he was better at it than Santino, but he was not above raising his voice, pounding tables, slapping his wife around, yelling at Tom about not giveing him straight answers anymore, yelling at Frankie about a shooting where his wife sleeps, being so vindictive to Kay about not seeing her children, to name a few. Vito never raised his voice, and never lost his cool. I think in III when Michael is advising Vincent about how Sonny's temper did him in he is also talking about himself and how his loss of cool has "clouded the judgment." This is not so much failure to take his father's advice but it is a character flaw. Come to think of it, all Vito's children had temper issues. Sonny for sure, Fredo could lose it, Connie was a basket case, and Michael as stated.

Great point DonTomasso. Another peice of advice Michael forgot was...

"A man who doesn't spend time with his family can never be a real man."

As we saw in Part II, Michael spent very little time with the Family, which led to his [and the Family's] downfall.

Mike forgot a lot of stuff in part 3

I think he was suffering from amnesia.

It was the diabetes.