Rarely, a Mac can experience a memory leak which causes MacOS to run out of memory. When this happens, it’s fairly obvious because not only does the Mac become unresponsive, but you see an error message on the screen that says “Your system has run out of application memory. To avoid problems with your computer, quit any applications you are not using.”

This error message is presented inside of a Force Quit window, where you can see what apps are open and using the most amount of memory.

Let’s discuss what causes the Mac to run out of application memory…