I maximized the game window right when minecraft was switching between the "building world" screen and the actual world, and this happened. (images below). Returning to the main menu does not fix it. I didn't try resizing the window, but quitting and restarting works. Also, everything else works fine, it just seems to be a rendering issue. No crash log, because no crash.
It is running forge, but i checked and it did it in vanilla as well.
No big deal, but I thought I'd report it anyhow.