Sheila Hynes, 72, died days after an operation at the Freeman Hospital in Newcastle in March 2015 to replace two heart valves.
The hospital admitted one of the valves had been inserted the wrong way round.
Surgeon Asif Shah told an inquest he had to reinsert the new aortic valve after a stitch cord snapped.
He said: "The procedure was going OK until the very last moment when I was tying down the mechanical valve and the stitch cord snapped."
Mr Shah, who the inquest heard had carried out more than 350 open heart operations since starting at the Freeman in January 2015, said he gave the valve to a nurse to hold as he readied to re-fit it.
The surgeon said the valve was then placed on its mounting the wrong way round - something he said he was unaware could happen.
Two attempts were then made to re-start Mrs Hyne's heart but both times tears were found in a ventricle and she suffered massive internal bleeding.
Later during the day-long surgery Mr Shah asked for seniors colleagues' help, at which point the valve error was first considered.
He said a senior colleague said he looked tired, and told him: "You've had a long day, you just go and have a cup of tea, take a break and I can replace the valve."
Mrs Hynes, a great-grandmother, had complained of shortness of breath and the operation to replace the aortic and mitral valves was aimed at relieving her symptoms and extending her life.
Mr Shah said the operation had been brought forward because Mrs Hynes' grandson was getting married later that year and her sister was terminally ill.
The inquest continues.
A cardiac surgeon whose patient died following an operation said he did not know a heart valve could be put in the wrong way round.