This case has two main findings:

• obvious right upper lobe pneumonia
• less obvious free gas under the right hemidiaphragm

How are the two findings related? The patient was 24 hours post-operative for appendectomy, and had developed respiratory symptoms, most likely an aspiration pneumonia given the history and that the right upper lobe is affected. 

This case demonstrates a potential satisfaction of search error, where the on spotting the right upper lobe pneumonia further abnormalities are not sought.