Improve FAQ formatting and add another question

docs/faq.rst

@@ -3,8 +3,8 @@
 Frequently Asked Questions
 ==========================
 
-* If I use :func:`~injector.inject` or scope decorators on my classess will
-  I be able to create instances of them without using Injector?
+If I use :func:`~injector.inject` or scope decorators on my classess will I be able to create instances of them without using Injector?
+---------------------------------------------------------------------------------------------------------------------------------------
 
 Yes. Scope decorators don't change the way you can construct your class
 instances without Injector interaction.
@@ -27,3 +27,41 @@ Frequently Asked Questions
 
     # will work
     X(s='a')
+
+
+I'm calling this method (/function/class) but I'm getting "TypeError: XXX() takes exactly X arguments (Y given)"
+----------------------------------------------------------------------------------------------------------------
+
+Example code:
+
+.. code-block:: python
+
+    class X(object):
+        @inject(s=str)
+        def __init__(self, s):
+            self.s = s
+
+    def configure(binder):
+        binder.bind(s, to='some string')
+
+    injector = Injector(configure)
+    x = X()
+
+Result?
+
+::
+
+    TypeError: __init__() takes exactly 2 arguments (1 given)
+
+Reason? There's *no* global state that :class:`Injector` modifies when
+it's instantiated and configured. Its whole knowledge about bindings etc.
+is stored in itself. Moreover :func:`inject` will *not* make
+dependencies appear out of thin air when you for example attempt to create
+an instance of a class manually (without ``Injector``'s help) - there's no
+global state ``@inject`` decorated methods can access.
+
+In order for ``X`` to be able to use bindings defined in ``@inject``
+decoration :class:`Injector` needs to be used (directly or indirectly)
+to create an instance of ``X``. This means most of the time you want to just
+inject ``X`` where you need it, you can also use :meth:`Injector.get` to obtain
+an instance of the class (see its documentation for usage notes).