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Several collections.Counter methods return Counter objects, which is leads to wrong or at least confusing behavior when Counter is subclassed. For example, nltk.FreqDist is a subclass of Counter:
>>> x = nltk.FreqDist(['a','a','b','b','b'])
>>> y = nltk.FreqDist(['b','b','b','b','b'])
>>> z = x + y
>>> z.__class__
<class 'collections.Counter'>
This applies to __add__(), __sub__(), __or__(), __and__(), __pos__(), and __neg__(). 
In contrast, the copy() method does (what I think is) the right thing:
>>> x.copy().__class__
<class 'nltk.probability.FreqDist'>