It is noted that even though Rose said that Bernard went to the bathroom before the plane crashed, he was found in a seat with the belt buckled. This is not a goof. It is possible that when Bernard went to the bathroom, on his way out the plane was crashing and so he grabbed the first available seat. It is unlikely that the writers would ignore such an obvious fact, that Bernard is Rose's husband. It should also be noted that this is what happened with Charlie when he went to the restroom near the front of the plane and then quickly buckled into a nearby seat as the plane began to crash.