Despite the shy mention in the docs, it was not clear to me that the future returned from asyncio.run_coroutine_threadsafe is not compatible with asyncio.ensure_future (and other asyncio functions), and it took me a fair amount of frustration and source-code-digging to figure out what was going on.

To avoid this confusion for other users, I think that a verbose TypeError warning when a concurrent.futures.Future object is passed into asyncio.ensure_future would be very helpful.