One word - trust.
All Flynn had to do was from day one lay out, directly to Pence and Trump, the entirety of his conversation, including it’s ambiguity. Trust would have been established and the ambiguity of the conversation would satisfy a diplomatic defense for it.
THAT - generating trust, was Flynn’s error. He was in one of the kinds of positions where that trust is of paramount importance.
I honestly don’t think he said anything wrong. It sounded like generalities to me to just hold off before retaliating (which obviously the liberals the to retaliate) and that things can be discussed in the new administration and perhaps they can forge a better relationship. The most the media could say was that he gave the “impression” that the sanctions may be lifted. He never said they would be, and they weren’t. In fact, Haley was out there saying they wouldn’t be until Russia is out of Crimea.
That said, I think this could be bad for Trump for the reason that the media is reporting that Sally Yates told Trump about this last month - several weeks ago, yet he didn’t make an issue of it until the media did, and supposedly just now Pence feels lied to. The media is now on a witchhunt. If he hadn’t of let him go, he could have said he didn’t think it was a big deal and that’s why he kept him, but now that he has, they want to know why he waited.