In the middle of a Reddit AMA, ostensibly about a Humble Bundle, lies a little piece of magic. It's a gallery, shared by Fez programmer Renaud Bédard, showing how the game looked when the code went a little wrong.
The one up top was probably "caused by declaring 16-bit indices and actually feeding 32-bit indices". The one below is not a C64 port of the game. It's "Lighting and instancing troubles with MonoGame at the beginning".
There's more here.
Fez glitches [gallery]