I often have trouble tracking the mouse pointer on the screen, especially the default small one. When I loose track of it, I bring it back at the top left corner of the screen. The first time I did that on the Mac, everything became black. Even if I was moving the mouse pointer or hitting a key, nothing was happening. Screen reappared after a few seconds. This happened a few times, but the other times, it was possible to leave this mode by just moving the mouse.
I figured out two things. First, the screen would become black every time I bring the mouse pointer at the top left corner. Second, the machine was so overloaded that at start, things didn’t respond properly, even exiting from this black mode.
I looked into the system preferences for a solution about this. The preferences are accessible from the Apple menu, available from all applications.
I found something quite interesting in preferences for Exposé and spaces: configurable hot corners.
Top left corner was configured to trigger the screen saver! AH! I clicked on this and that popped a menu offering me different options, including one to disable this unwanted (at least for me) behavior.
In conclusion, not a bug, just a feature. At least, this is configurable, as opposed to intrusive GNOME3 hot corners which cannot be tweaked from any GUI, at least in the versions I tried.