xrefresh
A program that makes the entire X display repaint itself by covering everything with a window and immediately removing it.
The X server generally doesn't keep the contents of obscured windows. When something is uncovered, the application gets an Expose event and is expected to redraw that part itself. xrefresh exploits this directly: it maps a window covering the whole screen and unmaps it straight away, so every window on the display receives an Expose and repaints. -solid gives the cover a colour, -root uses the root window's background, and -none makes it invisible.
The need arose whenever something had scribbled on the screen without asking — a misbehaving driver, a kernel message on the console, a program that crashed halfway through drawing an XOR rubber band. It assumes every client handles Expose properly, which is exactly the kind of assumption that left some of the debris in place anyway.