In thousands of games I never had this happen until today... someone started observing a 1v1 game (26688488) that I was playing. I hit "ready" at the end of the game, as I often do for a rematch. After I did so, the observer did the same, and we ended up in a 3 player game (26689068). I suspect this was intentional on the part of the observer, but I can't know for sure. Either way, I ended up in a 3 player game without expecting or desiring that.