Shawn

Since I have no knowledge as to the scope of your org, my suggestions were more to prompt you to think (ah yes, we did change X in PROD and forgot to include in sandbox).

As to the other points

• Different Sobject model (such as different formulas in formula fields; different RSF rules) - Where exactly would i go to compare the two?  Also is there any clue which one it would be or do I have to look at all? See Setup | Customize | sobject | Fields or Setup | Create | Sobject | Fields, - I would look at only those relevent to the Sobjects within test transaction in question
• Diferent sharing model - See Setup | Security | Sharing Settings - I have check the list and these are the same
• Different validation rules (something fails/passes in PROD but does the opposite in Sandbox because rules are different) - Not sure what you are referring to here, can you tell me where to go to look at these See Setup | Customize | sobject | Validation rules or Setup | Create | Sobject | Validation rules, - I would look at only those relevent to the Sobjects within test transaction in question
• Different workflow rules and field updates that cause actions on the database (again, in PROD, one set of actions, in sandbox, a different set) - Where exactly would i go to compare the two?  Also is there any clue which one it would be or do I have to look at all?  Setup  | Create  | Workflows (or Field Updates) - I would look at only those relevent to the Sobjects within test transaction in question

As for the code you posted

Your error message implies a problem creating a BeazerLot_Close which I'm guessing occurs as part of some trigger that executes during one of the DML statements below (probably the one: update lot1 after setting lot1.status to 'Closed').

// Create test lot, referencing the above account, but is not closed
BeazerLot__c lot1 = new BeazerLot__c(name='TEST 1', status__c = 'Not Closed', accountname__c = acc1.id);
insert lot1;
// Ensure no lot relationships were created
List<CC_LotRelationship__c> lot1Rels = [SELECT id, accountname__c FROM cc_lotrelationship__c WHERE BeazerLot__c = :lot1.id];
System.assertEquals(0, lot1Rels.size());
lot1.status__c = 'Closed';
update lot1;

// Should now have 4 lot relationships
lot1Rels = [SELECT id, accountname__c, accountname__r.firstname, accountname__r.lastname FROM cc_lotrelationship__c WHERE BeazerLot__c = :lot1.id];
System.assertEquals(4, lot1Rels.size());
lot1.accountname__c = acc2.id;
update lot1;

// Should now have 5 lot relationships
lot1Rels = [SELECT id, accountname__c, accountname__r.firstname, accountname__r.lastname FROM cc_lotrelationship__c WHERE BeazerLot__c = :lot1.id];
//System.assertEquals(5, lot1Rels.size());
delete lot1Rels;
lot1.accountname__c = acc1.id;
update lot1;
// Should now have 2 lot relationships
lot1Rels = [SELECT id, accountname__c, accountname__r.firstname, accountname__r.lastname FROM cc_lotrelationship__c WHERE BeazerLot__c = :lot1.id];
//System.assertEquals(2, lot1Rels.size());
lot1.accountname__c = acc2.id;
update lot1;
// Should now have 2 lot relationships
lot1Rels = [SELECT id, accountname__c, accountname__r.firstname, accountname__r.lastname FROM cc_lotrelationship__c WHERE BeazerLot__c = :lot1.id];
//System.assertEquals(2, lot1Rels.size());
}

 so the problem lies in that trigger (code not disclosed yet) and how it comes to the decision as to how to set the ownerId field. It is within that logic that there's a difference between sandbox and PROD