An (nowadays) open sewer in Herculaneum that illustrate the danger of holes in the pavement. The authorities has placed not only bars over it, but also a fence around it. The event below can be dated to between 158 and 138 BC.
"In my opinion then, the first to introduce the study of grammar into our city was Crates of Mallos, a contemporary of Aristarchus. He was sent to the senate by king Attalus between the second and third Punic wars, at about the time when Ennius died; and having fallen into the opening of a sewer in the Palatine quarter and broken his leg, he held numerous and frequent conferences during the whole time both of his embassy and of his convalescence, at which he constantly gave instruction, and thus set an example for our countrymen to imitate."
Suetonius - De Grammaticis II
Translated by J. C. Rolfe