When an application programme quits normally, the user is prompted to save any documents that have newly inputted or modified changes which have not yet been saved. The user can then choose to save or not to save the changes, and a large majority of programmes also offer an option to cancel the quitting process.
When an application, however, is stuck, or stops behaving normally, the user cannot quit the application normally. As a result, in order to force the programme to quit, the user is left with the sole option to force the application to quit. This would not allow unsaved changes to be kept; such changes are only stored in memory, to be gone once the application is force quit.
Classic Mac OS
Because the Classic Mac OS cannot support pre-emptive multitasking, any attempt to force a programme to quit would result in a general destabilisation of the whole system.
As of Mac OS 8, the dialogue box that appears once the Force Quit command is invoked includes a warning that force quitting an application will destabilise the system, and urges a restart once a programme is forced to quit.
Mac OS X
Mac OS X is a fully pre-emptive multitasking operating system. Force quitting applications in Mac OS X, therefore, cannot destabilise the entire system. Instead, only the application that was "acting up" is affected or taken hostage.
As of Mac OS X, applications can be force-quit using a Force Quit command in the Apple menu. As of Mac OS X 10.3, adding the shift key to the command-option-escape keystroke forces the current application to quit.
Forced Force Quitting
Under the following circumstances, a user may be obliged to force an application to quit, even when the program is not acting up or is being unresponsive:
Classic Mac OS as of Mac OS 8: A red dialogue box may appear, which alerts the user that there is no more free memory available. If the user clicks Restart, all applications are potentially forced to quit, and the system is restarted nearly instantaneously.
Mac OS X: Under very rare circumstances, virtual memory can eat away all memory, leaving no more free space on the hard drive. The only way to continue working is to quit an application -- and apps have to be force quit this way.