<p>Python 3.0 breaks backwards compatibility with previous versions and splits the language into two paths (at least temporarily). Do you know of any other language that went through such a major design phase while in maturity?</p>

<p>Also, do you believe that this is how programming languages should evolve or is the price to pay simply too high?</p>

C# and the .NET framework broke compatibility between versions 1.0 and 1.1 as well as between 1.1 and 2.0.  Running applications in different versions required having multiple versions of the .NET runtime installed.
At least they did include an upgrade wizard to upgrade source from one version to the next (it worked for most of our code).