Our worst leak was caused by a nearly catastrophic issue that was created even before we owned our previous boat, an Islander 32, way back in 2000 or so. The previous owner had his yard put new batteries aboard in the spring, a pair of group 31 deep cycles. What was done wrong and I'm not sure how it could have happened but the installer took a cable off the + side and bolted it to the thru bolt that attached the skeg! We had committed to buying the boat and the surveyor was aboard for the trip to the boatyard before hauling it to be sent to Maine.
When they hauled it up in the slings the strut was almost completely gone, the surveyor was shocked as well as the boatyard as this was the same place that had commissioned the boat. They fixed the damage of course, I should point out that the boat was on a dock and plugged into shore power, essentially he had been pumping some serious voltage into the water at his dock, the surveyor said that if the boat had sat on the dock another couple of days it would have sunk right there!
The saga continues as when we got the boat everything seemed good, strut was repaired and the battery wiring was done correctly. Over the remainder of the season, we took delivery in mid August, we had no issues.
The next season I noticed some pretty serious bilge pump activity, I checked the packing box and while it was dripping I didn't think it was unusual. Over the course of that season it got worse, and worse. After a more than cursory inspection I noticed that the stern tube was leaking badly, right where it went through the hull. That became my priority project that winter and what fun that was! after I got the stuffing box and transmission coupling apart the tube broke off, kind of like a loose tooth, remember those? Wiggle, wiggle plink.....so we were lucky we didn't loose it.
No pictures but it was scary.