A woman was taken to hospital after being given a coffee containing cleaning fluid at Waitrose in Lewes.
Waitrose confirmed the “unfortunate incident” occurred at its Lewes store earlier this month.
Paramedics attended the supermarket in Eastgate Street in the town centre and advised the woman to attend a specialist burns hospital after she drank the coffee which she received for free through Waitrose's membership card scheme.
The supermarket giant has apologised for the incident but would not confirm if the woman received compensation, claiming it was a private matter.
However, the woman's husband, who has not been named, said on an internet forum Waitrose had offered his wife £25.
He said his wife spent hours in Royal Sussex County Hospital's accident and emergency after the incident.
In the post, he said: “She had a coffee from the machine in Waitrose and maybe they hadn't rinsed the machine out so she ended up drinking a cup of coffee containing cleaning chemicals and then we both spent the morning in Brighton A&E.
“She's still in some discomfort but will probably be OK.”
A few days later he added: “Well five days later my wife is still unwell and we just received a letter of apology from Waitrose with a £25 voucher.
“Am I the only the only one who thinks that's not really compensatory for the injury and suffering, having to spend hours in A&E, missing a weekend away etc?”
A Waitrose spokeswoman said: "We are very sorry that there was an unfortunate incident in our Lewes branch on March 8 when a customer accidentally received some very diluted cleaning fluid in her hot drink.
“Our branch first aider immediately attended the customer and, as a precaution, called an ambulance.
“We have confirmed that the cleaning fluid does not have any long term impact on health, given the small quantities consumed, but we are very sorry for what would have been an unpleasant experience.
“We have taken action to ensure that this doesn't happen again and will be talking directly to the customer to apologise for this distressing experience.”
A South East Coast Ambulance spokeswoman said paramedics attended the store at 8.30am, adding: “We were called to a woman who suffered a reaction in her mouth after drinking coffee which was thought to contain a cleaning solution.
“An ambulance was sent and the patient was assessed and treated on scene before being advised to attend Queen Victoria Hospital for further advice and treatment.”