If the window is disabled mid-screen via rLCDC and WX+7 lines up with SCX & 7 (the specifics of how to describe this are unclear to me), a pixel with color value of 0 and the lowest priority is pushed onto the background FIFO. I've only observed this on DMG/MGB. It doesn't appear to occur on CGB in DMG mode. It's also been observed on GBBP.

The attached ROMS can be used to reproduce this in various ways. It's also been observed on the title screen of https://gearfo.itch.io/corrib75.

Note: As of this writing I'm unaware of an emulator which reproduces this bug.

window_disable_bug.zip