The prohibitory sign is a sign of trouble on a Macintosh running Mac OS X. It has the same meaning as a Sad Mac icon and the Chimes of Death on a pre-OS X Mac, meaning that it signifies trouble (the Mac is unable to boot normally).

The prohibitory sign is a large, deep-grey in colour circle with a diagonal line in the middle of it, alongside a lighter grey screen.

Under Mac OS X, the opposite of the Prohibitory sign would be a grey Apple with an animation beneath it.

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