Consider this program

def foo(a,b):
	return min(zip(a,b)[2])

print foo(range(5), (0,9,-9))

With the default options, 2to3 rewrites this as

def foo(a,b):
	return min(zip(a,b)[2])

print(foo(list(range(5)), (0,9,-9)))

For some reason, 2to3 fails to wrap the call to zip in a list, even though the 2to3 documentation says that there is a fixer which does this. Obviously, the generated code will not work in Python 3 since you can't subscript an iterator.