"Fredo, you're my older brother, and I love you. But don't ever take sides with anyone against the Family again. Ever."

In my opinion, the way Michael paused and emphasized the word Ever in that scene really portended that Fredo would eventually betray him. Of course I'm saying this with the benefit of hindsight. I was 13 years old when I saw "The Godfather" in the theatre in 1972, and I honestly don't recall if I remember thinking that the first time I saw it.

My question is this: Does anyone think it was deliberate on the part of Puzo/FFC? Is it possible that they were thinking of Fredo's betrayal before they even finished filming the first movie?

I know it's kind of a silly topic, but we're running out of material here .

I think it's stated that way to emphasize Michael's evolution into the familiar character we know in the Trilogy. The evolution contrasts his estrangement of his family ("That's my family Kay; it's not me")to his cloaking his actions in his family ("ever").

A more fundamental question is whether a sequel had even been contemplated at this point. But if so, then I agree with you that they did a good job of dropping the hint.

There's a very similar line in the book: "Freddie, you're my older brother, I have respect for you. But don't ever take sides with anybody against the Family again."

Extremely unlikely. The novel was written and Part I was filmed without thought of making a sequel.

Part II was made possible by ultra capitalist turned movie lover Charles Bluhdorn.

Thanks Charles!

Should we thank Bluhdorn or Robert Evans?

It was Charles Bluhdorn who wanted a second Godfather after seeing the first. He basically begged Francis to make another one. It was him who arranged for the crew to film the Cuba scenes in the Dominican Republic.

I think it was the very first time that Michael has to poignantly exclaim that he is now indeed head of the family - period. If you recall, Fredo makes a plea to Tom Hagen and tries to change Michael's decision. Tom tells Fredo that Mike is in charge now but it seems to fall on deaf ears. It showed the audience that Michael was not going to be swayed or disrespected by anyone... even his own brother.

Interesting difference between novel and film: In the novel Moe Greene is still present when Michael says this line. Which seems to me like a tactic mistake, similar to Sonny's and Vito's mistake in the discussion with Sollozzo.
Fredo & Sonny differ from the leader. Michael & Vito point it out when the enemy is present.
In the movie it makes more sense to me that Moe's already gone.

I think Oli and the Professor nicely summarized the answer to PB's question.

To Danito's point: Michael's rebuking of Fredo in Moe Green's presence could have been meant to diminish Fredo in Moe's eyes. Moe was already walking all over him (when not slapping him around), but he thought he had Fredo so thoroughy cowed that Fredo could be an ally against Michael. Never happen again after that rebuke. Also firmly put Fredo in his place.