On a somewhat related note.. Tell me what you think of this:

I was thinking about how Sollozzo and Bruno Tattaglia knew not to trust Luca Brasi when he comes to talk to them, and they end up killing them. My theory is that Barzini had a hand in that, too. Notice how Barzini is at Connie's wedding at the beginning of the film. I bet he saw Luca there and realized he was loyal to Vito and wouldn't betray him. So, when Luca comes to the Tattaglias and pretends to be unhappy with the Corleone family, Barzini probably tipped them off that he was lying.

I realize it's a stretch of the imagination, but think about it.