I've got a 2004 Yamaha F225 on my boat with around 1k hours. I had the lower unit dropped and the mid section inspected when I purchased it, of which the dealer said it looked great, and without any issues at all. As alluded to above, it seems like it's something that CAN happen at just about any point, though not that it always WILL. I've heard, and I don't know how accurate all of these stories are, that it doesn't necessarily reflect upon maintenance or flushing the engines or anything as it's a dry exhaust thing, not from where the water runs. I'm somewhat of the opinion that it might just be bad castings from a certain time frame and from a certain location where the outboards were built. Like on my previous boat, it had a Honda 130 hp four stroke on it, and they had a recall on several serial numbers where the castings on the blocks were affected because of an issue. Fortunately the motor that I had wasn't one of the numbers that was affected, and I checked that beforehand. But again imo, and while I don't have any proof of this, it seems like it might be something similar on the Yamaha where there were maybe some bad castings on the dry exhaust area. Otherwise, I'm not sure how some people have the issue whereas others don't, especially when a lot of people who have had the issue state that they are religious on maintenance and/or take it to their mechanic every year. Then on the other hand, I'm sure there's been a good deal of people who lack doing much, if any maintenance at all, that likely have never had the issue and probably didn't even know that there was one.