If you have a lot of visual force pages on force.com sites, one of the challenges is fixing the error has occurred error message on each visual force page. This message would drive you crazy and takes most of the time for developers to fix it. So here is some common tips where you can fix issues quickly.
1. If a visual force page crashes with the error has occured, one area to look is at the on load method on the visual force page. Go to the controller on load method and look at the new custom objects you have added in the soql query. Make sure that the guest user profile has read access to the custom object and field security is set to visible for those fields used in the soql query.
2. Another area is to look the logged in user profile. You can see this on customer portal settings where you can check the customer profile and for each profile, ensure that the custom object has field security and read permission on the custom object.
So these 2 places would help you to fix most of the security issues on visual force pages on force.com sites.