A poorly toddler with suspected meningitis lay on a makeshift bed of two chairs while he waited for several hours in A&E to be seen by a doctor, his mother claims.
Rose Newman, from Eastbourne, East Sussex, told The Mirror her son Jack, one, was forced to wait for five hours because of scarce resources after he was rushed to Conquest Hospital in Hastings.
The incident was raised at Prime Minister's Questions on Wednesday by leader of the opposition Jeremy Corbyn.
Mr Corbyn recounted how a woman called Sian who works in the NHS and has a 22-month-old nephew saw the boy treated on two plastic chairs pushed together with a blanket.
He said: "Does the Prime Minister and the Health Secretary think this is an acceptable way of treating a 22-month-old?"
Mrs May replied: "I accept there have been a small number of incidents where unacceptable practices have taken place.
"We don't want those things to happen, but what matters is that the NHS looks into issues where there are unacceptable incidents that have taken place and then learns lessons from them."
Ms Newman told the Mirror: "As a first-time mum, it was really scary. The fear we had, the not knowing what was going on, was crazy.
"Theresa May said she accepted there had been a few instances where things like this happen. That is laughable."
Of the hospital, she added: "They were brilliant when we were there and the staff wanted to help, they really did, you could tell, but they just couldn't. They don't have the resources."
East Sussex Healthcare, the trust responsible for the hospital, said: "Jack was recorded as attending A&E at 8.40pm.
"Jack was assessed by a clinical practitioner as soon as he arrived.
"It was decided that Jack did not require a bed. He and his mum were put in an assessment cubicle which is a seated area within the Emergency Department.
"During this time Jack was monitored by the nursing team and given paracetamol for his temperature. Jack's mother laid him down.
"Jack did wait over three hours to see a doctor due to the volume of other patients being seen. Following a medical assessment Jack and his mum returned home.
"The cubicle they were put into does not have a bed as it is for assessment and not treatment of patients.
"Had it been clinically necessary for Jack to be admitted to a bed in the hospital, this would have been done."