Not quite dialogue, but it turns out the colleague I've been calling Sid for the last 18 months is actually called Simon & and I only found out when his wife called and asked to speak to Simon, and I told her nobody called Simon works here...
Something very similar when I was around 19 or 20. We had a new office boy start, straight from school.
We were told he was arriving and as I happened to be the first one in I naturally said hello and asked what his name was. 'Bill' he told me.
As the office filled up I introduced him to the other office staff as Bill. Everyone called him Bill.
A year later, maybe more, we found out his name was Alan. Alan Beal.
He either was to shy to say anything or thought we were all a right lot of snotty b'stards.