Cheatin’ uh? error message on stock theme

Hello,

A client of mine has a site built in wordpress using a theme purchased on themeforest. Since a recent upgrade to version 3.5 part of the functionality built into the theme no longer works and the Cheatin’ uh? message appears. I have found various solutions to this error message on wordpress but none I can find to apply to this particular theme which is now no longer available on themeforest. Would anyone be able to shed some light on what exactly is the root cause of this Cheatin’ uh? error message so maybe I can find a fix with this theme?

Many thanks.