A wagon containing propane was found to be overfilled while it was being weighed just after arriving at a refinery. The wagon had left an LNG site on a train of empty wagons without first being emptied. In addition, more gas had been added to the wagon. No signs of leaks or physical damage were found on the wagon. Two technicians (one of whom was a trainee) were in charge of emptying three wagons at the same time. Usually, only one technician is required. The technicians forgot to open a valve on the wagon, causing the event. The checks performed at the gas terminal to make sure that wagons are indeed empty failed to detect this error.