I've had this problem too, usually rectangles of black appearing but also occasionally other images or text appearing where they shouldn't. It's not consistent, I might get it on a page but then revisiting that page later it won't happen.
Seems to be display related as if I flip to another tab and then back to the problem page the problem usually goes away and the problem page is redrawn correctly.
I'm using Opera 33.0.1990.115