I created the following validation rule on my contact records:
IF( ISNEW()&& ( FormAssembly__c =""&& TEXT(LeadSource)="") && TEXT(Contact_Source_List__c) ="",TRUE,FALSE)
I went to validate a new custom object and fields that has a master-detail relationship to contacts. I got an error that two Apex tests had failed and that I don't have enough test code coverage (never had this issue before). Both errors referred to the validation rule. I tried deactivating the validation rule and then validating the inbound change set again to test, and it succeeded with no problem. Hence, I'm pretty sure the validation rule was the cause of the original failure.
I'm still a newbie with code, so I don't really know how to go about troubleshooting this! Any help would be much appreciated!