I wonder if it's a variation of this...
Move the mouse or trackpad cursor over the center area of the login window so you can see the user icons. Click on the icon of the user that you would like to login as, type in the user's password, and press Return.
If the login window is configured to show only the name and password fields, type in the user's name and password into the fields, and press Return (even if you cannot see the rest of the login window).
This issue will not occur if the display is not sleeping when the account is logged out. Use the steps below to confirm that the account is not configured to log out automatically while the display is sleeping:
- Open System Preferences > Security & Privacy > General. Click the padlock to unlock the preference pane and enter your admin password. Click the Advanced button at the bottom, then see if the option "Log out after N minutes of inactivity" (where N is the number of minutes) is enabled.
- Open System Preferences > Energy Saver and configure Display Sleep to occur after the account is logged out, by dragging the slider to a number of minutes that is greater than N was set to in the previous step.
Important: If automatic log out is not needed, disable "Log out after Nminutes of inactivity" in System Preferences > Security & Privacy > General. This will also prevent the issue.
Do you mean that everything is displayed with a light blue tint? I have the same problem with my MacBook Pro, but never when I wake up.
I found a workaround from the user "Umputun" in this discussion: Blue tinted screen after mountain lion
See my reply in that discussion for details about how it works for me. I'm not sure if it'll solve you problem, since it happens for you when you wake up your iMac.
John In Paris, BDAuqa--
No, there's no logon window to see and it's not a light blue tint, it's solid blue. I have the same problem on a new Mac Mini with Mountain Lion. Reminds me of the "blue screen of death" on a PC, except it's blank where the PC had a "you're screwed" text message.
I've only been a Mac user for a year and a half. I'm disappointed to be encountering such problems. If I wanted problems I would have stuck with the PC. At least there I could fix them.