Carry On My Wayward Son

One time a serious promise was broken was when Dean broke his promise not to bring Sam back to life again  on Supernatural. Dean and Sam have been in a topsy-turvy relationship since season one and have on several occasions brought each other back from the dead. however, every time they save one another, they end up putting their own lives at risk, such as the one time dean sold his soul to save Sam. In season eight, Sam was trying to close the gates to hell, but completing the trails proved to be too challenging and they almost killed Sam. After stopping the trails, Sam still found himself in critical condition, and eventually ended up in a coma. Sam's pending death made Dean super sad, so he made a deal with an angel (thinking it was Ezekiel when really it was Gaderial) that allowed the angel to enter Sam's body and act as a life support. In the end, breaking this promise was harmful for everyone involved (especially Kevin).