Cheatin uh? error

Hi,

I am trying to help a friend with her web-page by changing the theme so it looks better then what it does now. I chose to download Optimizer and activate it in my wordpress but when I try to preview it or use it I get the error message “Cheatin uh?”. I’ve done some google searches on it but the error can be very theme specific as you have to change something in the code for it to work. I am the administrator and I’ve tried de-activating the plug-ins to see if it helps.
Is there anybody that knows why this error message comes up and what I can do about it?