Misprinted Bill with Retained Obstruction Affecting Printing Area
Retained obstruction misprints are very collectible, and quite rare. On the example below a piece of brown paper got in the way of the third printing. Below the brown paper will be a blank area where the printing was missed.
Retained obstructions are rare because usually whatever caused the obstruction was quickly lost. The stranger the obstruction is, the more valuable the note will be. One of the most famous retained obstructions is the "banana" note. This was created when a Del Monte banana sticker got stuck on a bill during the printing process.
There is no easy guideline for pricing obstruction errors. It is all about eye appeal and degree of error.