That could be the reason for interview, if they ask you about it tell them the truth that you forgot to mention it, they would already have it on their system. Did u apply on ur own? or thru lawyer? Nevertheless, take all proofs specially post marriage proofs like emails, pics, chats, any trip which ur spouse might have made after applying. 1 of my friends hubby too was refused 2 times student visa and they had mentioned it on the application, but he was still called for interview. I dont know if their interview is complete, will let u know once she lets me know.