This first happened months ago (and has not disappeared) but today I finally decided to post it here.
Every time I start the game, all is good. But when I open the "city map" view, I get a red error bar in the middle of the screen. It sais: "Context3D error: Error #3685". When I go to a different screen (when I want to loot a building, for example) the error stays on the screen, blocking a small part. I can still play, but it is quite annoying.
The error only disappears when I refresh the page, but since you have to go to the map before you send your survivors on missions, I can't raid anything without that stupid error bar.
I hope this can be fixed.