I have been having this issue in my flash games with Opera 26.0 for the last few days as well, it ran fine before for the most part. Now when I tab out of my flash games, I am unable to go back into the flash window and interact with anything within the flash window. I've found that right-clicking in the flash window and opening up "Inspect Element" or changing my Opera from full screen mode to windowed or vice versa enables me to interact within the flash window again until I tab out again. I have always had this tab-out issue come up with my flash games in Opera, but it happened only rarely until recently, and tabbing out used to fix it not worsen it.
I also noticed that the right-click drop-down menu in the flash window is different when it's being unresponsive: the options are Save As..., Print..., and Inspect Element. When the flash is running and interacting appropriately, the right-click drop-down menu includes: Version Number, Server End, Settings, Global Settings, and About Adobe Flash Player. I am currently using Opera 26.0, OS Windows 8.1. I love Opera for it's speed, as my flash games require speed.
Per this thread, I'm trying out Opera 27, and it seems to be working for now. I have not yet had the tab-out issue discussed.