Vito's slip ups, believing among others -

• the minting money Narcotics deal offer was just between Sollozzo, Tattaglia and the Corleones only
• Barzini and the other families wouldn't want a piece of this lucrative deal - 30% share alone should be three, four million dollars just in the first year
• Sollozzo would just accept Vito's refusal No repercussions
• As we'd debated before - perhaps biggest slip up was sending Luca over to Tattaglias which nobody Tom, Sonny, nobody knew about, thus to Luca's death, losing the Corleones feared Enforcer, one man killing machine

Also, Vito
1. didn't up his personal protection
2. didn't smell a rat when Paulie was calling in sick
3. had Fredo -- well -- Fredo was -- well – as his Bodyguard / Driver

As regards Sonny's death, I doubt anyone would have seen it coming from Carlo
Also as Tom said "Things are starting to loosen up a little bit" as well perhaps lulling everyone into a false sense of security

side question please
Why would Sonny think Tattaglia's [who are way down in the pecking order] guarantee is a big deal?