Send Two Return One Watari

Common with beginners.  

I think the "common with beginners" reference here is if Black fills at . However, it depends on who is komaster for . Let's investigate some of Black and White's responses.

This fails.

White tries a ko (5 at )  

is tesuji. White cannot connect because of oiotoshi, and even if he did connect, he cannot capture the capturing stone because of .

White tries to capture (5 at , 6 at )  

If W is komaster, White should send two and let Black return one, then block on the side. Black connects, White pushes once.

White success  

If B is komaster, then White should play this way: