Boots has been fined £10,000 after a 76-year-old man fell through an open trap door at one of its shops in Powys.
Martin Habberly needed hospital treatment to his leg after the incident at the Welshpool store last May.
Wrexham magistrates sitting in Mold were told the trap door had been opened to allow meters to be read in a cellar.
The hearing was told the trap door was "unique" among Boots' 2,500 UK stores, and it admitted breaching health and safety regulations.
Magistrates heard that the open trap door should have been guarded by a member of staff.
But the court heard that at the time of the incident, an untrained shopfloor employee had become distracted by another customer.
Nigel Vaughan, prosecuting on behalf of Powys council, said Mr Habberly was looking for various items in the shop when he walked around a member of staff and fell into the open void.
He fell down three steps, sustaining serious gashes to his leg.
A safety barrier was available to fence off the area, but had not been used for some time, the court heard.
Admitting failing to assess risks at the store and a health and safety offence, the Boots defence team said the issue had been taken very seriously by the company.
David Charlton, defending, said senior management in Boots had not been aware that the store had a trap door that could be opened in public areas during shopping hours.
Following the incident, he said measures had been taken to ensure it could not happen again.
Barriers are now used, with trained staff in high-visibility jackets. The meters in the cellar are also in the process of being moved.
Boots has apologised to the victim and a civil claim for compensation was due to be resolved now that medical reports had been received, magistrates were told.
"Boots is a responsible company which has pleaded guilty following what is a genuinely isolated and unfortunate occurrence from which the company has learnt its lesson," added Mr Charlton.
The company was fined £5,000 each for the two offences and ordered to pay legal costs of £1,240.