Drop a POP-up is a term used to describe a type of computer user interface. It is a window that appears on the screen when a user clicks on a certain area or button. The window usually contains additional information or options related to the area or button that was clicked. The term 'drop a POP-up' comes from the fact that the window appears suddenly, as if it were dropped onto the screen. The answer to the crossword clue ERR is POP.