Here are some facts in hopes that it will help alleviate some stress with anyone who experiences this error:
There is a problem currently where sometimes, once a player has completed a game (note: completed) win or lose, people get a message saying "You have forfeit this game" (paraphrasing)
The reason you are experiencing this incorrect message is because when the game completed, you closed your browser instantly. When a game is done, you must click through the post-game screen until you get to the main menu. At that point, you can exit WGT without fear of receiving this bug.
Fortunately, experiencing this bug does not affect your profile in any way. You won't get a loss on your record, you won't lose additional credits, and your completion percentage will not change. This is purely a messaging bug and there is a fix on the way in early October.
Thanks for reading and for your understanding while we work out the bugs. :)
Stay classy,
-WGTpizza