It all happened during the recording for '2017 Idol Star Athletics Championship'. First, Sehun is captured casually chatting with labelmate NCT 127's Johnny.
Sehun walks off to the side for a moment...
..and returns thinking that Johnny is still there, and starts talking to him again, but OOPS! That ain't no Johnny! It was actually Seventeen's Junghan that Sehun was casually starting a conversation with. Once he realized his mistake, Sehun jumps up in surprise, exchanges an awkward head bow, and quickly walks off.
Currently, netizens are giving thanks to Sehun for the good laugh.