Mac Client allowing you to be in multiple games at once

I play on a Mac, and usually i have no problems, however sometimes if i end a game on a bad note, and tab out before clicking on the defeat button, then come back to my computer and forget that the game is still technically in session for me, it gives me the option of starting a new game. So i can load into 2 games at once, which causes me to go afk from the start of the game. i have to reboot client. (im currently loading back into a game right now because of this). Thank you