Almost all of my repair jobs start with the owner :
forgetting it, not installing it, simply placing it on the outdrive. Once dis-lodged water enters the bellows.The resulting damage is usually the gimbal bearing and U joints.Depending on the time spam and salinity the transom housing corrodes away where the clamp goes for the bellows.
The boats serviced with the ring glued in place with bellows adhesive or 3M Weatherstrip adhesive never have this problem.
Another area is the water inlet O ring. Dis-lodged or a poor seal can force water into the bellows area.
And over the years I have found lots of shortcuts to the repairs but not gluing the ring in place is not one of them