William Crozier was caught out after parking in the Alexandra Road car park, which used to be free.
He had previously received just two parking tickets in his whole life - but after being caught out three times in a week earlier this month, he has been slapped with £300 worth of fines.
The Second World War airgunner said: “There are a lot of people out there who have been ticketed.
“I’ve paid all three of them because I don’t know what they’re going to do if I don’t.
“It’s £100 per fine so that’s £300 I’ve had to pay and it’s out of order, it’s ridiculous.
“I’ve only had two tickets in my life, it’s a rip off.”
Charges at the former Co-op store’s car park had not been enforced since it shut in January, with many using the 150-space car park for free.
In March, new signs went up and an ANPR system was installed by new operators Smart Parking.
But the Northants Telegraph has since been contacted by dozens of shoppers who have said there was no indication they would be charged and that they hadn’t met a grace period.
A Smart Parking spokesman said: “Smart Parking recently changed the tariffs at this car park.
“Ahead of doing this we instilled 29 new signs which cleanly state the new terms and conditions of use.
“This new signage is markedly different to the old signage in both colour and design.
“In addition, Smart Parking initiated a three-day crossover period before the new tariffs come into effect.
“We would remind any motorist to always check the terms and conditions of use at a private car park.”
Angus Gill of the British Motorists Protection Association said he had been in touch with some of the drivers from Corby.
He said: “Firstly this hijacking of motorists by suddenly changing terms and conditions is actually in breach of the code of practice that Smart signed up to with the British Parking Association.
“They have a specific clause which covers this. The “grace” period has been suggested as one month and not three days to allow this familiarisation.
“Where there is any change in the terms and conditions that materially affects the motorist then you should make these clear on signs.
“Where such changes impose liability where none previously existed then you should consider a grace period to allow regular visitors to the site to adjust and familiarise themselves with the changes.
“Secondly, local drivers tell us the mandatory entrance signs - again required by the Code of Practice - are not there or were not there when Smart started to turn on the cameras.
“Lastly, it is frequently the case with most parking companies, though we have not been able to verify this, that they fail to apply for planning permission to have the new signs in place.
“And occasionally there will be restrictions within the existing planning permissions that limit what companies like Smart can do.
“Since most of the profit for these companies relies on catching drivers unaware, drivers must appeal to Smart and remind them they have responsibilities to make people aware of these changes.
“Perhaps your readers caught out by these signs will remind them.”