Aside from a) the Mac OS to Mac OS X transition (though existing apps worked under "Classic" mode), b) some apps during the PPC to Intel transition (namely apps with assembly or endian issues), and c) apps that used private / unpublished APIs, that's not really true.

It's true Microsoft goes to extraordinary lengths to keep backwards compatibility. Consider the Joel Spolsky anecdote about SimCity: http://www.joelonsoftware.com/articles/APIWar.html

In case you missed that, SimCity had a bug where it used memory it already freed. This worked under DOS for whatever reason, but not under Windows. So the Windows team hardcoded a special case just for SimCity.

That is going too far, IMO. Why should my OS have all this extra bloat to check for buggy applications? This is why OS vendors seed preview releases to developers, so they can test their own damn software and issue fixes for these kinds of bugs. While it may have been hard to distribute updates 20 years ago, with the internet it's trivial.