Am I sinful for having gone out with my friends after my father refused and I feigned his approval through my mother's consent?

Previous fatwas have clarified that obedience to parents is obligatory in matters where there is a benefit for the parent and no harm to the child. If a father forbids his son from going to a certain place for a valid reason, such as fearing religious or worldly harm upon him, then obedience is obligatory. However, if that is not the case, there is no sin upon the son for going. It is always preferable to obey the parent, as long as they do not command disobedience to Allah.