2005 (and related year/models) Honda Civic
The air filter housing on the 2005 Honda Civic has retainer nuts inside the top piece of the air filter housing that may fall out during air filter removal. Use caution not to spin air box screws out too far, thus pushing out the retainer nut. If the retainer nut is pushed out of the air box, it may fall into the throttle body (intake) located at the bottom of the lower air box.
Here are some pictures of the air box and retainer nuts in question:
