Known small pulmonary arteriovenous malformation (pAVM) in left lower pulmonary lobe (LLL). Presented to the emergency department with fever. Chest x-ray on arrival was normal.
 12 hours later, developed severe neck pain and showed signs and symptoms of meningitis. Complete blood count and chemistry panel came back normal. Extensive testing for pathogens all came back negative - putative diagnosis of aseptic meningitis. Had grand mal seizures. CT head and MRI brain (not shown) were normal.
A consecutive chest x-ray revealed a new ovoid opacity in the LLL. CTPA showed it to be the pAVM, only it had grown substantially. The seizures were thought to have caused its rapid growth ¹.

NB: I contributed the images for the reference article.