All passengers had already boarded Monday's Germanwings flight to Zurich, but were told to leave the plane after a technical problem was identified.
The Kölner Stadt Anzeiger (KSTA) reported on Tuesday that ground staff directed the passengers to buses which were supposed to take them to a replacement plane for the journey.
All but one of the bus drivers were told which plane to take the passengers to - but the last bus, with 12 people on board, just waited at the gate for instructions. The passengers had a good view of planes taking off and landing - but no chance of getting on their flight.
“Eventually the bus driver began to wonder, because he hadn't been told to move,” Markus Goller, who was on the bus, told the KSTA. The driver then went inside to ask staff what was going on, before driving to the runway.
But by the time the passengers got there, there was no plane. It was already on its way to the Swiss city. “We just stared in disbelief,” said Goller. His and the other 11 passengers' luggage was on the flight.
A spokesman for Germanwings told the paper it appeared there was a muddle over which bus was carrying the "boarding completed" sign - only the last bus in a queue should display it, but it seems this was not the case.
Those who missed their flight were offered a seat on a flight to Zurich the next day, and a refund.
“We can only apologize to everyone,” said spokesman Heinz Joachim Schöttes. “It was a series of unfortunate events."