Matt - Wow! speechless....
Guessing he also framed the niche?
Brian - thats too funny - I had a guy several years ago I caught using cardboard to fill in low spots on a floor

One corner of a room dove off about 2" in about 8 feet from the corner
When I asked "WTH are you doing?"
He said "Don't worry - once the cardboard sucks the portland outta the thin set it will be hard as a rock"
I said "You mean like the rocks in your head? - I want you to take all of that back up & redo it ....without the cardboard."
He said "I don't redo any of my work"
I said "Take your tools & git!"
Once I got it pulled up - not only did he have cardboard under there - in the deepest spot in the corner he had a 1x6 buried in thinset!
