This happens when, during your print, your screen stops displaying relevant information and instead displays a series of black boxes and strange hieroglyphic-esque symbols.
The reason for this may be that the SD card inserted into Palette™/Palette+™ was bumped, or something may have occurred within the electronics of the screen.
However, there's good news – Palette/Palette+ is still working! The screen may not be displaying the most relevant information, but your Palette/Palette will continue to create filament like it normally should. The screen will return once your print is complete, and you are able to restart your Palette/Palette+.