According to a broadcast affiliate on May 2, Jo In Sung participated in the 500th episode recording on April 29.

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The segment 'Happy Together Friends' is set to return in light of the 500th episode, where celebrities get a chance to connect with their old friends via a phone conversation as well as a face-to-face.

It appears no one was aware of Jo In Sung's appearance as it was not scheduled ahead of time. The actor decided to appear on the show thanks to MC Yoo Jae Suk.  

The 500th episode will air on May 25!