Hi to all :)--> and sorry if I interrupt the discussion. Forgive my English too, it is not my mother tongue, Greek is.
I just want to share a thought.
Didn't Adam and Eve died? Well the animals (goats/sheeps/lions whatever...) from which God made the skins did. Isn't that substitution? (either rituals and animal sacrifices for the Jews, or Jesus for Christians - Something/Someone dies instead of you)
It was the same day, God did not lie. someone died.
Doesn't this explain Cain's wrong offering?
I always wondered how on earth could poor Cain knew that he needed a blood sacrifice, since the law was given years later, even centuries. (off-topic -excuse me)
As for the initial question of what was the sin, I agree with those that said disobedience.
If I say the expression right:" that's my 2 cents"