I created a simple Matching Rule in my sandbox called
ReplayedReceipts on object
Receipt__c that did an exact match on a single field
There were no records of
Receipt__c in my sandbox.
SFDC said my matching rule couldn't be activated:
You tried to activate the matching rule ReplayedReceipts for identifying duplicate records. However, we can't activate the rule because it defines an unusually large number of records as possible duplicates. Specifically, take a closer look at these fields:
Edit ReplayedReceipts to include a field that contains more unique values, and then try activating the rule again.