Send Two Return One Watari
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.
is tesuji. White cannot connect because of oiotoshi, and even if he did connect, he cannot capture the capturing stone because of .
This threatens to capture all of the side Black stones.
If B is komaster, then White should play this way: