What actions ignore field level security in Salesforce?

Formula fields can ignore the Field-Level Security. For example say you created a field called Number Field on Opportunity of type Number with access permissions to the System Administrator. Also you created an another field called Formula Field on Opportunity of type Formula returning a Number and configured the access permissions to both the System Administrator and the Standard User. This formula field evaluates to Number_Field__c / 2.

Now, by the permissions the Standard User should not be able to read and edit the Number Field on Opportunity. But, the Formula Field on Opportunity will still show a value of say 500 if the Number Field on Opportunity had a number say 1000 which was earlier created by an another System Adminstrator.