I have been to every room, all gold. I have every artifact and all of the red pieces. However, somehow, I have avoided the last bit piece. I've been to the owl room also. Does anyone have any ideas/experience for this? Or is my save file doomed/glitched

It's been a while since I've played Fez but on the map screen there should be a picture of a shard next to the room it's in. If not then look for rooms with a question mark next to their picture.

My last piece was definitely in the 4-owl room.

If you are triple-sure that no room on the map is black or has the shard icon, I don't know what to say. Perhaps the next patch will shake it lose? :(

My friend had this problem. Enter the achievement code again, see if you spawn another cube. He accidentally walked out of the room without grabbing the anti-cube.

EDIT: @Evansoft