Hospital bosses say staff have been left "devastated" after a premature baby died following "gross failure" in his care.
He was described as a "miracle" by his parents having been born at 28 weeks and weighing less than 2lbs following six rounds of IVF treatment.
An inquest heard how a feeding line, called an umbilical venous catheter, had been incorrectly inserted in his abdomen.
A consultant then forgot to reposition it, leading to a fatal build up of fluid around his heart.
'We are so very sorry'
After a narrative conclusion by the assistant coroner Abigail Combes, the Sheffield Teaching Hospitals NHS Trust issued a full apology.
Medical director Dr Jennifer Hill said: "We know that no apology will ever be enough to lessen the pain of Cassian’s death for his parents and family, but we are so very sorry for what happened and have already provided Mr and Mrs Curry with a full explanation of what happened and the changes we have made since his death."
Cassian's parents raised concerns that understaffing over the Easter bank holiday weekend were partly to blame for what happened, but Dr Hill said staffing was not an issue.
She said: "Whilst staffing numbers on the neonatal unit that weekend were appropriate and within national recommendations it was very busy and regrettably there was a genuine human error in terms of the management of Cassian’s umbilical venous catheter.
"This was a very rare incident to have happened and everyone involved in his care is devastated. There has been a full review of what happened, and changes have already been made to limit the chances of this happening again including additional consultant support at weekends and ongoing improvements to the documentation used.
"We will also be taking on board any further recommendations from the coroner and ensuring we respond with appropriate actions."