rentals and stations (each station's neighborhood, number of docks and opening year) are loaded for Harbor Bikes. They share no column name: rentals have start_station and end_station, stations have station. When the key is named differently on each side, use left_on= and right_on= instead of on=.
Two more edge cases appear here. Joining the same table twice (once for where a rental started, once for where it ended) creates clashing column names, which suffixes= resolves. And a survey file stores rental ids as plain numbers (5001) while rentals uses R5001: a merge between text and numbers fails until you make the formats match.