Ryanair is to investigate after a family had to pay a £55 penalty for excess luggage which weighed in at less than their total allowance.
The fine took the shine off the Scotts' holiday in France
The Scott family of four, from St Ives, Cambs, had 50kg of luggage in two cases instead of packing it into four when they flew to France from Stansted.
But returning from Carcassone they had to pay the penalty because each case exceeded the allowance for one person.
They have since learned that sharing allowances is not permitted by Ryanair.
In a statement Ryanair said: "This may be an isolated incident, but we are investigating."
Return leg Mandy Scott said the airline's policy was unclear and she is demanding a refund.
Mrs Scott also wants to know why they were not told the rules when they flew out, and were only penalised on the return leg of the journey.
"When we questioned it we were told it was in the terms and conditions but when you read it, it really isn't clear at all," she said.
Her husband, Jes Scott, suggested Ryanair's ban on pooling baggage allowances may have been introduced because the airline charges passengers who book via the internet £2.50 for each item checked in.