A friend had a mechanic work on his boat and didn't check the mechanic's work. The mechanic had left the coupling bolts loose and they all worked out except one, at which point the center of effort was no longer even close to the point of alignment. The action caused extensive fiberglass damage in the shaft log area and the boat started to take on water pretty quickly. (Fortunately he was near a port and was able to get towed and hauled in time to prevent sinking. Unfortunately it was at the start of a family vacation and they lost most of the remainder of the boating season while wating for the boatyard and insurance company.