class Foo(object):
 
     def __init__(self):
         print "foo"
         raise RuntimeError
         yield 23

 Foo()

With a classic class, this correctly fails saying
"__init__ must return None".  With a new-style class,
the class suite works, and Foo() is called without
reporting an error, but also without printing "foo".

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You don't have to be that cute:

>>> class Foo(object):
...  def __init__(self):
...   return "foo"
... 

where did that check go?

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Albertas Agejevas has pointed out that:

1. New style classes don't care what is returned from __init__.

2. If __init__ contains a yield, the code in __init__ won't
be called until the generator that is returned has its
'.next()' method called.

Which all makes sense.  However, someone I work with was
trying for ages to work out what a particular __init__
method was not being called.  The __init__ method was rather
long, and there was a yield, copy-pasted from some other
code, hidden behind a raise.

So, the behaviour may be correct, but I don't think it is
very helpful. 

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Fixed.
See typeobject.c 2.264 and 2.264.2.1.