One of my ex's had a nephew, and we were minding him one day, and as we were tapping around a football in the back garden, he started telling me all these things about his deceased Grandfather, I'm not talking about trivial things, I'm talking personal things, which only my ex's Grandmother would have known, not even his children.
I had just assumed this was all common knowledge, so just continued the conversation.
Anyway, later that night we were sitting down, myself and my ex, and we were just chatting and I brought it up, "I didn't know your Grandfather did 'x', 'y' and 'z' ", my ex looked at me as if I had a severed head and asked my what I was talking about, so I told her. She sort of laughed it off with a "You know what he's like", so I thought no more of it.
However, a week or so later we had all gathered at my ex's Grandmother's house, just for a family get together, and the young lad brought the topic up again, and similar to my ex's reaction, her aunts and uncles laughed, however, her Grandmother's face was a different story.
Turns out that what he said was absolutely bang on, and some of the stuff he came out with, was stuff only the Grandmother knew.