Reproduction:
1) Create a document with a boolean field named :empty. Set field to true.
2) Enable raise_not_found_error
3) Try to locate this document using Model.find (fails).
4) Disable raise_not_found_error
4) Try to locate document using Model.find (document magically reappears from the dead!).

Cause:
mongoid/contexts/ids.rb raises DocumentNotFound if raise_not_found_error and result.blank? are true. If the object responds to "empty" with "true," blank? also returns true, resulting in a DocumentNotFound error even though the document exists. With raise_not_found_error disabled, this faulty blank? logic is never triggered.