Chris's Wiki :: blog/tech/MistakesAndBackwardCompatibility
Plenty of people like backwards compatibility, especially strong backwards compatibility. But it has what is a sometimes unpleasant consequence, which is that a strong commitment to backwards compatibility requires keeping your mistakes. Or at least many of them. To put it one way, you need to keep mistakes that work, and of course you have to keep them giving the same result as they currently do. For example if you provide an API that people can use to express potentially conflicting things and you don't reject the attempt but instead give some deterministic result, you're stuck with it.
You don't have to have a strong commitment to backwards compatibility, of course, and many people think it's better not to. Microsoft Windows is a big example of such a strong commitment (what has sometimes been called 'bug for bug compatibility'), and while it's given Microsoft a lot of commercial success it's also left them with a lot of technical challenges that have required heroic (and expensive) work to deal with. Backwards compatibility in general is definitely something that helps some groups at the potential expense of others, and it has a cost regardless of whatever benefits it gives to people.
But if you're going to make a strong commitment to backwards compatibility, it comes with the warts, including that your mistakes have to be preserved. If you don't want to have to preserve your mistakes, you should be honest about the limits of your commitment. You don't have to do this (you don't have to do anything), but if you don't it can surprise people and make them unhappy.
