among others -
1. Sonny letting Sollozzo know, Sonny was hot for Sollozzo's narcotics deal
2. Sonny was a hothead, can't talk business with him yet Vito would have made Sonny Don if Sonny had not been killed
3. Sonny also taking the risk visiting his goumar Lucy's apartment during the gangland warfare
Tom not digging deep enough to unearth the Barzini connection with Sollozzo's narcotics trade deal
Tom obviously stopped when Tom unearthed the Tattaglia connection and looked pretty pleased with his handiwork when Sollozzo said “My compliments”
Then Vito would have upped his personal protection and been ready for Barzini too
Tom letting his one and only client, Michael commit perjury because Tom had no idea Pentangeli had survived and was the secret weapon being prepared in custody, to corroborate Michael's murderous, nefarious power as the head of the Corleone crime family
Tom also had no idea Roth was returning to America after being refused refuge in various other countries
oh, well! Tom was not a wartime
Consiglieri
