The only check that prevents instantiating abstract classes is in object.__new__, but most built-in classes never actually call object.__new__. That means you can do stuff like

import abc

class Foo(list, metaclass=abc.ABCMeta):
	@abc.abstractmethod
	def abstract(self):
		pass

Foo()

and the Foo() call will silently succeed.

Ideally, the Foo() call should fail. Other options include having the Foo class definition itself fail, or just making a note in the documentation describing the limitation. (As far as I can see, this is currently undocumented.)