This one will get thousands of votes....I recently had a project that got me wrapped around the axle....geodatabase replication wasn't working...days of trouble shooting...turns out I had populated a related table with external data that had Version 2 of the GUID format......ESRI products get wonky with anything other than Version 4. It would be handy if there was a GUID validator GP tool....yes, I know, I can code this with python. Like I said..."Handy".
