You should have seen the face of the United Airlines agent at Sao Paulo last week when I asked for a seat assignment change (moving to an open center section of five seats at the last minute) and she pulled up my itinerary. She noticed that I did not have a departure stamp on my boarding pass and her eyes opened widely when she saw my itinerary. She said a few words in Portuguese to her colleague seated next to her then asked me, "You're flying from Washington to Chicago via Brazil?"
I smiled and said yes, though I explained I was actually supposed to be in GRU for the 12-hours, but the mechanical delay on the flight I just stepped off changed those plans. The woman laughed and said, "If you say so" before printing my new boarding pass.
Good thing the question came from her and not U.S. immigration authorities.