If you are using a force.com site and having a lot of visual force pages, you would encounter an error message which is error has occurred suddenly when you edit or roll back changes on visual force page. This would drive you insane and make you to pull your hair.

Cause of the problem.
Some times if you have apex classes which are dependent among them, making a change in one visual force page or class can result in a compile exception. If you ignore the error which happens when you use IDE like eclipse or force.com ide, the ide would say page compiled good. So this is caused to incomplete compile errors.

Solution
1. Go to the setup–>develop –> apex classes and click on compile all classes.
2. This would compile all classes and spit out the compile error. You can then fix the code and recompile which would solve the problem..Hope this solution works good for your site.