When I forget to put the computer (iMac OS X 10.7.4) to sleep and return several hours later, why does dragging the mouse generate grey pixilated lines on the screen?

Have you looked to see what power settings you have—i.e., what default behavior is in place for the computer’s behavior when it has been idle for some specified amount of time?

I have an energy saver panel in “Preferences’ but that mentions nothing about the mouse. Unless I do not understand you, which is more than likely. I did reset to default now and will see what happens.

Thanks.

Mouse movement is activity. When you move the mouse, it’s no longer idle.

Is it a laptop and did you close the top?
My Macbook Pro sometimes will not “wake-up” if the top has been closed down.

@Jeruba; I went to the energy saver page on System Preferences. I reset the various options (that I had been fiddling with) to the default. And lo, the problem has disappeared.

Thank you for the advice. Logically, I don’t understand, but that is irrelevant.

Apple tech has solved this problem. I had to go to the Advanced tab in “Security and Privacy” in System Preferences, unlock it, and turn off the “automatically log out in xx minutes” box.”

I was having the same problem with the cursor on the Macbook Air screen, with the same solution.