This paper revisits the experiences with the use of formal methods in the development of the control system for the Maeslant Kering. The Maeslant Kering is the movable barrier which has to protect Rotterdam from floodings while, at almost the same time, not restricting shipping traffic to the port of Rotterdam. The control system, called BOS, completely autonomously decides about closing and opening of the barrier and, when necessary, also performs these tasks without human intervention. BOS is a safety-critical software system of the highest Safety Integrity Level according to the IEC 61508 standard. One of the reliability increasing techniques used during its development is formal methods. This paper revisits the earlier published experiences with the project after the system is in operation for ten years and has performed its first autonomous barrier operation on November 11^th, 2007.