Probably when Sung Ryong was going through his 'Only I can take him down' phase.
Choo Nam Ho: So you are telling me when you think of Seo Yul your chest feels tight and your face gets hot. And you think it means you are angry.
Sung Ryong: Yes.
CNH: Do feel like this when you are mad at me?
SR: No.
CNH: Do you feel like this when you are mad at literally anyone else?
SR: No.
CNH: So what you are feeling towards Seo Yul isn't anger. It's most likely...
SR: *Light bulb going off* Super anger.
CNH: *Kicks Sung Ryong off the couch* *going to sleep*