Here is my version of the Trailhead Valication Rule:

AND (
NOT (ISBLANK(  AccountId  )),  
MailingPostalCode <>  Account.ShippingPostalCode   )

I tried it out and it works fine (checked syntax and also tried to create a contact with a different zip than the account and got the error, corrected the zip and the contact was saved).  But I get this error in Trailhead.

"There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, Contacts associated with an Account must have the same Mailing Postal Code as the Account Shipping Postal Code: [MailingPostalCode]"

Am I blind and missing something?????