Q: "Your computer needs to restart" message after period on Sleep?

For the second time in the last couple of weeks, I have opened up my MacBook Air after a period of several hours on Sleep, and found the screen shows a message in five or six major world languages reading "Your computer needs to restart because of a problem", or something to that effect - this on a dark central box on top of the desktop image. The message continues by saying that, if the machine doesn't restart automatically, I'm to use the power button to restart it.

I am unable to go around this message, although I must admit it doesn't look like the kind of Apple-quality message screen I'd expect, aesthetically.

The machine then restarts after a few seconds, and seems to function okay after this, but I've never seen such a message on a previous Mac, and wonder if it suggests a problem with the machine.

Thanks for any advice. My machine is about 18 months old.