Add footgun section.

Author: ZeroIntensity

peps/pep-9999.rst

@@ -288,6 +288,40 @@ intends to be very intuitive; users should be able to deduce the behavior of
 ``yield from`` in an asynchronous generator based on their own background
 knowledge of ``yield from`` in synchronous generators.
 
+Potential footguns
+------------------
+
+Forgetting to ``await`` before ``yield from``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In :mod:`asyncio`, a :ref:`future <asyncio-future-obj>` object is natively
+iterable. This means that if one were trying to iterate over the result of a
+future, forgetting to :keyword:`await` the future may accidentally await the
+future itself, leading to a spurious error.
+
+For example:
+
+.. code-block:: python
+
+    import asyncio
+
+    async def steps():
+        await asyncio.sleep(0.25)
+        await asyncio.sleep(0.25)
+        await asyncio.sleep(0.25)
+        return [1, 2, 3]
+
+    async def generator():
+        # Forgot to await!
+        yield from asyncio.ensure_future(steps())
+
+    async def run():
+        total = 0
+        async for i in generator():
+            # TypeError?!
+            total += i
+        print(total)
+
 
 Reference Implementation
 ========================