I am trying to create a validation rule which checks whether Field A OR Field B or no field is filled in. Basically, we want to disallow that both fields are filled in at the same time. I have created the below validation rule, but it gives me an error when I try to leave both fields blank. Is there an easier way of expressing this?
OR( AND(checkbox__c , NOT(ISBLANK(Lookup__c))), AND(NOT(checkbox__c), ISBLANK(Lookup__c)), AND(NOT(checkbox__c), NOT(ISBLANK(Lookup__c))))