MS has only been good at backwards compatibility when it is in their best interest. Usually because a major partner was the one using an undocumented API with millions of users. In this case it was a person hacking some code together on a product less than a month old with probably only a few thousand people using the app that broke.
Microsoft is bad at many things, but until recently, they were very good at backward compatibility. They knew people were using undocumented APIs, so they went out of their way to preserve bug-for-bug compatibility in a lot of cases.
On the other hand, they also went out of their way to use undocumented APIs and API changes to give Word an advantage over Word Perfect....