I was reading through the RFC for the finger protocol (RFC1288, from dec. 1991), and thought this an interesting inclusion:
2.5.5. Vending machines
Vending machines SHOULD respond to a {C} request with a list of all items currently available for purchase and possible consumption. Vending machines SHOULD respond to a {U}{C} request with a detailed count or list of the particular product or product slot. Vending machines should NEVER NEVER EVER eat money.
This section was absent in the original RFC (RFC742, from dec. 1977), which was an incredibly succinct document. It was, however, present in both RFC1194 and RFC1196 (both from late 1990), with a slight variation on the final sentence:
Vending machines should NEVER NEVER EVER eat requests. Or money.