Earlier today, I was watching the Mural Painting 16 segment on YouTube, and I saw something peering from behind the 6, with a few straight lines. I looked in the comments and saw someone say they just slapped a 6 over the 15 mural. Then I saw a picture of the 15 mural segment on Muppet Wiki, and saw what appeared to be the remains of a 4 behind the 5...
And then it hit me, these segments all used the same mural, just lazily slapping a new number on it (although, I haven't checked if this remains true with 17-19, as those used a different template from 14-16).