Validation rule triggered when ownership changes

Validation rules will fire with single ownership changes in Lightning Experience

• For example going to an account record, clicking on [change] next to the owner field. You will then be presented the change ownership page. When you attempt to save the ownership change, it will evaluate all validation rules on the account object. The validation message will appear on the change ownership screen below the lookup to user box on save. 
• If there is a billing address validation rule that city cannot be null and the record was created before the validation rule, ownership change will evaluate that rule and not allow you to change the ownership of the record until the record meets all validation criteria.
• This also includes changes made via the data loader or updates via the API, since changes are made at a single record level. 

What if I don't want the rules to fire for all users? 
 

(What if my admins are the ones doing the owner change and they do not have the data needed to pass the validation rule?  How can I allow them to make the change?) 

• If there are some users who do not have to comply with the validation rules, then write the validation rule in a way that excludes them, using the $Profile.Name global variable. 
• For example, if the billing address is usually required, but not for the standard admin profile, then the validation formula would be:

See Also:
Validation Rule Considerations  
Lock the Opportunity Product with validation rules once the Opportunity Stage is closed