OK, so we have a Widget version 1.0 which works with our PBX version 1.0 (lets say).
If you upgrade your PBX to version 3.0 from 1.0, Widget 1.0 no longer works correctly. You MUST upgrade to Widget 3.0, but in order to do that you have to upgrade to Widget 2.0, then Widget 3.0.
How does the customer do that? No one knows because the guy responsible for the Widget product has not bothered to come up with a Widget migration path.
Nice, so now the customer sits with a Widget paperweight until someone comes up with a plan.
Fortunately, that someone is NOT me.