[Photograph: jonjk on Flickr]

Foolishpoolish just directed me to this photo and discussion on Flickr. The photographer writes:

we had no idea what we were doing, but the miffed lady we got sat next to flipped her unfinished pizza upside-down when she was done eating, so well, uhh, when in rome! (in retrospect, we think it was a sign that we didn't like the pizza, which wasn't completely true [it was kind of burnt], but the lady was quite upset at the service she [didn't] get.)

Someone commented on the photo:

flipping ones pizza is one of the strongest of all possible types of italian non-verbal communication

Sounds like the commenter was pulling the photographer's leg. But who knows. Maybe this is a gesture of grievance? If so, I know some pizza nerds who have been inadvertently flipping the bird, figuratively, to pizzamakers.