Programmer's key

The programmer's key (also known as the interrupt key) is a keyboard combination in classic Mac OS where simultaneously pressing the command key and power key can be used to invoke the MacsBug console or the built-in mini debugger (known as "MicroBug"). Using this key combination during bootup on an Old World ROM system will result in a Sad Mac error.

History
Classic Macintosh models through the Power Mac G4 included small programmer's interrupt ( ⎉ ) and reset ( ◁ ) switches hidden near or on the back of the computer. These switches can be pressed with the end of a paper clip. Early models included a clip-on accessory to make it easier to access these switches. As the clip-on buttons disappeared from later models, the keyboard combination could be added as an alternative through an extension (INIT) written by Apple engineer Paul Mercer for System 6.0.8 and earlier. System 7 through Mac OS 9 have this key combination built-in.

Discontinuation
The hardware switches were retired on Macintosh models that did not support natively booting into classic Mac OS. Mac OS X was released with different debugging features and no longer supported this key combination.

Articles

 * Happy Birthday, Mac - How to Recover From the Dreaded Bomb Box Error Message by Sandro Cuccia at The Mac Observer (2014-01-25)