First, What's New " explains the new features in Python". This issue is a bugfix. AFAIK, object() has always been documented as having no parameters. The fact that passing extra args should raise a TypeError is no secret.

Second, this *is* documented. The third sentence of
http://docs.python.org/3/whatsnew/3.3.html
is " For full details, see the changelog." We really mean that ;-). The changelog is derived from Misc/NEWS in the repository. It says "Issue #1683368: object.__new__ and object.__init__ raise a TypeError if they are passed arguments and their complementary method is not overridden." That is prefixed by "Issue #1683368:", which links to this issue. This entry is easily found by searching for 'object.__init__' (or a sufficient prefix thereof).

For 3.2, the What's New sentence was "For full details, see the Misc/NEWS file" and the link went to the raw repository file.
http://hg.python.org/cpython/file/3.2/Misc/NEWS
My impression is that this issue played a role in including the prettified version, instead of just the repository link, in the on-line version of the docs. What's New for 2.7 does not even have the link.

In any case, *any* bugfix breaks code that depends on the bug. Hence the decision to make the full changelog more available and more readable.

I realize that the change to the header for What's New is hard to miss. But what are we to do? Add a new What's New in What's New doc for one release? Put the change in flashing red type?