Sometimes, the whole display gets shifted one pixel down. This means that every pixel on screen is displayed one pixel lower than it should be and the bottom line of pixels wraps around and appears at the top of the screen.
This glitch appears during the normal development process and may have something to do with waking from sleep or USB data transfer. It's fairly rare (it happened to me four times so far) and never while doing anything "special" that I didn't do many times before.
This glitch is NOT tied to changing display brightness or resetting the watch.
Whenever the glitch happens, it's very noticeable (for example all menus lose their bottom border) and is persistent across all displayed screen and applications, until the watch is reset (then the glitch is fixed).