Years ago, after hiking in Manuel Antonio Parque Nacional in Costa Rica, Mary and I came down to the beach and saw a guy selling fresh-squeezed orange juice. We were parched with thirst, and Costa Rica, after all, is about the only place South of the Rio where you can safely drink the water. So I ordered a big glass, took it and chugged it with delight.
Then I watched in horror as the guy took back the glass - his ONLY glass - and gave it a quick swish in a bucket of cold water under his table before setting it up for the next customer.
There were no repercussions, but it was a mighty close call.
(And then there was the time I ate the mayo with my fries in a little rosticeria in Cuzco, but that gamble didn't work out so well ...
)