Bomb (icon)

Since the classic Mac OS offered no memory protection, an application crash would often take down the entire system.

Originally, the resume button was unavailable unless the running program had provided the OS with code to allow recovery.

With the advent of System 7, if the OS thought it could handle recovery, a normal error dialog box was displayed, and the application was forced to quit.

The debugger program MacsBug was sometimes used even by end users to provide basic (though not always reliable) error recovery, and could be used for troubleshooting purposes, much as the output of a Unix kernel panic or a Windows NT Blue Screen of Death could be.

A kernel panic screen (either text overwritten on the screen in older versions, or simplified to a reboot message in more recent versions) replaces the bomb symbol but appears less often due to the radically different system architecture.

Mac OS system error alert from the System 7 era. These were a common sight, and Mac users of the era might need a paper clip(if the Programmer's Key wasn't installed) in order to restart the computer since the onscreen restart button would usually be nonfunctional.
On the Atari ST, the four bombs indicate that the system error "Illegal Instruction" has occurred.