So while rewatching the episode "Make New Friends but 
FANMADE Mettatonex
Keep Discord", I noticed a character I had never seed before in hiding in the background in a part of the episode. She only was visible for about a second before she disappeared.

I took a closer look and I instantly recognized her as Mettaton EX from Undertale. Did anyone else watching  the episode notice her? Because I think it was a cool easter egg.