...you don't understand the "unfold everything" step.go, go, go
...you go to the store and ask "whats the largest piece of paper you've got?"
...you make a "HOW I MADE PAPERCRAFTS PLOZ" without reading two pages back to discover there are already a billion of the same damn thread.
...you over-score the glue tabs, and the damned <whatever you are building> falls apart...Of course, this has NEVER happened to me...
...you request an origami model that is either already on the board, or doesn't exist and will never be found. LIke that Gundam one that got /r/'d over... and over... and over... and over...
...you score folds with a scalpel... on lightwight paper