Now and then, I've meet people who said that one or another OS is uncrushable. They seem to believe that just because they haven't crashed their system at some point in time, it means that no one will ever be able to do so. This belief seems to be most spread on people using an Apple's OS.
Any OS can crash, but some are more stable and robust than other. By this I mean, than some operating systems are harder to crash because they are more resilient because of the way they where designed. In essence, under what operating load an OS will crash is determined by the design parameters.
If an OS is well designed, and used as intended, crashes shouldn't happen under normal circumstances. The only OS that crashed on me on regular basis has been Windows Vista, which seem to crash for no particular reason. Windows 95, 7 and XP did crash on me on from time to time. Yet, those times where when I did put to much strain on the system resources until the system just gave up.
Since I moved on to GNU/Linux, some 7 years ago, I've come along two systems crashes. One on Ubuntu and another using Linux Mint. On both cases, putting to much strain on system resources was the cause, not a fault of the OS it self.
On the Apple's operating systems, I've no first hand experience. Yet, I've seen people who use OS X on regular basis crash it from time to time. So, as with Windows and GNU/Linux, OS X can be crashed.
It seems to me, that GNU/Linux and OS X are a lot more resilient to system crashes than Windows. On all my experience with Ubuntu and Linux Mint I mainly have to worry about certain application crashing because I had another one eating too much RAM at the time, but most of the time it has been smooth sailing. And for the people who use OS X, it's pretty much the same thing, a rouge application crashing because of the same reason.
Yet, I wouldn't call any GNU/Linux or OS X crash proof. It's a matter of how you use your OS, and of time, before a full system crash comes along.