I worked for this place that made a security auditing tool. The agent had to know which version of Windows it was running on. When Microsoft came out with a new version, the agent didn't know what it was running on, since it hadn't been updated to include that version. I forget what it did in response, but it didn't run right - maybe just terminated?

Microsoft discovered this in their testing, before they released the new Windows version. In response, they added our agent to a list of programs that Windows would lie to about the Windows version. If our program asked, we would be told the previous version.

Well, that was great, in that our program didn't terminate early (or whatever it did). But now, even after we updated our agent, it still wouldn't know whether it was running on the new version of Windows, and so would not audit it correctly!

This dance shows just how far Microsoft went with backward compatibility. We were not a top-1000-most-used program by any means, but they knew about us and tested us and fixed for us. (I mean, we didn't want it, in this case, but it was still impressive.)