Prohibitory symbol

The prohibitory sign is a sign of trouble on a Mac running Mac OS X or macOS. It is similar to a Sad Mac icon and the Chimes of Death in classic Mac OS, which signifies that the Mac is unable to boot normally.

Mac OS X
In past versions of Mac OS X, the prohibitory sign appears as a large grey circle with a diagonal line across the middle of it, on a lighter grey screen. Normal booting would display a grey Apple logo with a spinning animation of progress beneath it.

macOS
In recent versions of macOS, the prohibitory sign appears as a white symbol on a black screen. Normal booting would display a white Apple logo with a progress bar underneath.