Market efficiency is not a Binary YES/NO variable.  Rather it is a continuum from not so efficient to very very efficient.  NOTE . market efficiency does not imply perfection! :) 

Links I mention: 

https://helpified.com/paths/technically-not-passive

and 

Wisdom of Crowds (short and funny)  https://www.youtube.com/watch?v=r-FonWBEb0o

On Warren Buffett's bet: http://fortune.com/2017/02/25/warren-buffett-scorches-the-hedge-funds/