An elderly couple were told they faced legal action unless they settled a rail fine, which they say had nothing to do with them.
A fair dodger on the Southwick to Lancing line gave their address after being caught without a £2 single ticket last month.
Two weeks later, Edward and Barbara Fisher received a letter from a recovery firm addressed to the parent or guardian of a name unknown to the couple.
The headed letter stated they must pay a fine of £15.80 within ten days or face appropriate action to recover the debt without further warning.
Mrs Fisher, 77, was stunned by the letter as the couple seldom travel by train, preferring to travel by car.
They suspected a youngster had been caught without a ticket and given rail officials a false address.
Mrs Fisher, from Portslade, said: "When I opened the letter, I was absolutely incensed.
"To think we are being threatened in this way for something that has nothing to do with us made my blood boil. My husband had only a few days before receiving the letter been released from Worthing Hospital after having a hernia operation so he could have done without this."
On the day the letter arrived, Mrs Fisher phoned the recovery firm, Revenue Protection Support Services, but reached an answering machine telling her to write if the address was wrong.
She sent a letter by recorded delivery to the firm's PO Box address in Portsmouth.
However, on Thursday, the couple received another letter demanding the £15.80, this time from Debt Recovery and Prosecution Services, with a PO Box address in Portsmouth.
It stated if payment was not received within ten days, legal proceedings would start to recover the debt plus an additional administration fee.
Mrs Fisher, a retired surgical appliance fitter, said: "Apart from being accused of something we haven't done, it's the sheer hassle of having to write a letter, go to the Post Office and pay for a recorded delivery."
The couple, who have a joint income of £120 a week, refuse to pay the debt.
A manager at the Revenue Protection Support Services was adamant they had not received a letter from the Fishers.
She said: "I can confirm the case but we have received absolutely nothing at all as regards to a letter.
"We are very stringent with everything that comes in. As soon as a letter arrives, it is immediately logged in our database.
"But in this case, I can say for sure that nothing arrived, which is perhaps why a second letter was sent out to the couple."
However, The Argus has learned from Royal Mail that the recorded delivery letter was signed at the firm's offices on October 25, a day after the Fisher's received the first letter.
A spokesman for rail company South Central said he would ensure the demand be dropped.
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