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Consider the following:
>>> class Test:
... def __init__(self):
... self.items = []
... def __len__(self):
... if not self.items:
... self.items = list(self.calc_items())
... return len(self.items)
... def __iter__(self):
... return iter(self.items)
... def calc_items(self, number):
... return range(1, number)
... 
>>> l = list(Test())
>>> print(l)
[]
>>> t = tuple(Test())
>>> print(t)
()
In the above example calc_items() method is called with a missing argument, which raises TypeError. That TypeError is being wrongly interpreted as "object of type 'Test' has no len()" and is swallowed by _PyObject_LengthHint(). 
The result is entirely unpredictable as the bug is masked, which is especially annoying for objects that can have a complex call graph under __len__(). Possible solution would be to adjust _PyObject_LengthHint() to rely on some other exception rather than straight TypeError. Swallowing a generic exception like that is bad.