An in-game character name switch

I started a two-player trade game on non-ladder non-hardcore server for an item I wanted to trade the other day. I always limit the number of players to join my trade games to one. A player with name "X" (not a real name) joined my game. As I was farming for uber keys at the moment I hadn't had a chance to send an in-game greeting message to the player as I saw a message player "Y" (not a real player name) left the game. That's right, the player's name who left the game was different from the player's name who initially joined the game. I grabbed a screenshot of the messages, but I can't share it on this forum as the site management doesn't allow any 3rd party names/nicknames to be revealed, so you'd have to trust me on this.

Any ideas for how the in-game character name switch could happen?

Addendum: I did try to /whois both character names soon after the player left the game, but no ID's have been returned. Apparently the player logged out.

Yes I've seen similar glitch on my own toons before.

I was on my mule A doing some inventory, then afterwards joined a game on toon B to meet up with some friends. When I used in-game chat, initially I was speaking as B, but all of a sudden it became A. My friends got confused and asked why A is talking while they see me as B.

It's the Matrix, man.

I have seen sometimes the Bnet tag mixed with the char tag.

I.e. "xxxx#1234 entered the game", while the game-char name was shown for talking or in the top-left icons.

But never saw char name changes. After that, I am no expert on public games