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.
Did it take long before the spectator joined to game? This might be a race condition between both of you hitting "Ready" at the same time and the info that there was another player in the game not being transmitted to you in time.
I think the problem is players joining doesn't reset the ready status?
For example.
P1 joins game
P2 joins game
P1 ready (I want to play a 2 player game) look away from screen.
P3 joins game
P2 ready
P3 ready
Game starts
P1 is now in a 3 player game having only wanted to play 2 player.