In Lion I always used Control ⌃+Shift ⇧+Eject ⏏ to put my display to sleep when I walked away from my computer. The system would continue running and not go to sleep.
Now in Mountain Lion when I put the display to sleep, the system sometimes decides to go to sleep as well.
It does not happen all of the time, it has been rather unpredictable. Some times I walk away for 2 minutes and come back and it went to sleep entirely. Other times I walk away for hours and come back and it did not go to sleep.
Very odd. Any ideas?
EDIT
Screenshots for both Power Adaptor and Battery Energy Saver settings.
Also I just experienced the issue again. I walked away for about 3-4 minutes and came back and WiFi was disconnected so it does not seem like the Energy Saver settings are being respected.
Unless… Energy Saver is considering the 15 minutes have elapsed being that my Display has gone to sleep and is therefore kicking in the computer sleep Energy Saver setting? I will up the Computer Sleep and see what happens.
Best Answer
Realised I answered a dupe of this here: How can I download something with display turned off (Ctrl+Shift+Eject)?
My answer to that question below:
I always use this combo as well, however this does not immediately sleep my machine in ML.
I believe this is a separate issue at play here. According to Siracusa's ML review, unlike in previous OS X versions, in 10.8 your system is eligible for sleep immediately upon "user inactivity" (the inactivity delay being set in your energy saver settings) unless the currently running apps make "power assertions".
In this case, that key combo is signalling to the OS that "user inactivity" should be triggered right away. In such a case, if your particular app that's downloading stuff does not make power assertions, your system will sleep.
I have come across the same issue generally (not necessarily related to triggering the screen sleep combo). See: Mountain Lion sleeps too aggressively