Apple acknowledged the problems on Sunday with a posting on its support forums suggesting the error may have been caused by a conflict with Unsanity’s APE (Application Enhancer) software for previous versions of OS X that some users had installed. After initially denying it on its blog, Unsanity admitted that older versions of its software were to blame.
Apple has since posted a fix for the problem on its support website, suggesting that users carry out an “archive and install” installation of the upgrade, which transfers existing OS X system files into a folder named “previous system” and then installs a new copy of Mac OS X on the selected volume. However, other Mac users have reported they’re getting the same error even though they don't have any Unsanity programs installed. If you are experiencing any of these problems, the guys at MacFixIt have compiled a useful list of tips to get you back to normality.