This title might seem a bit like a contradiction to the previous chapter…and of course, you really shouldn't break your API, if you can help it. But sometimes, maintaining backwards compatibility for any area of your application can lead to a point of diminishing returns. This applies to everything about a program, not just its API.
A great example of the backwards-compatibility problem is Perl. If you read the summaries of the perl5-porters mailing list, or if you're familiar with the history of the Perl internals in general, you'll have some idea of what I mean.
Perl is full of support for strange syntaxes that really, nobody should be using anymore. For example, in Perl, you're supposed to call methods on an object like $object->method(). But there's also a syntax called the "indirect object syntax" where you can do method $object. Not method($object) though – only the case without the parenthesis is the indirect object syntax...