by Most crashes on a Mac affect just one application. But you may encounter a type of system-wide crash that brings down your entire Mac: a kernel panic. When this occurs, there’s no warning and no way to save your work or do anything else without restarting. And, because kernel panics can have many different causes, diagnosing the problem and preventing its recurrence are difficult.
How do you know if it’s a kernel panic?
If you’re running OS X 10.7 Lion or earlier, kernel panics usually result in your screen dimming from top to bottom, and a message appearing in several languages telling you that you must restart your Mac (by holding down the power button for several seconds to turn it off, and then pressing it again to turn it back on).
Up through Lion, a kernel panic looked like this (on an otherwise unresponsive screen).
Starting in OS X 10.8 Mountain Lion, OS X automatically restarts when you have a kernel panic, and then displays a similar-looking message for 60 seconds (or until you press a key) telling you that your Mac was restarted because of a problem. (If the kernel panic repeats every time your Mac restarts, OS X will give up after five tries and shut your Mac down.)
As Apple notes on its support page about kernel panics, something as random and fleeting as malformed network packets can potentially cause a kernel panic. So, if you experience this problem just once, or only rarely, just restart, get back to work, and forget about it.
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