A health board is having to spend £200,000 on new surgical instruments after a second round of problems with equipment over the summer.
In June, 55 cardiac operations at the University Hospital of Wales in Cardiff were cancelled after sticky residues were found on instruments.
Managers thought they had solved the problem, but residues have returned, meaning further delays to surgery.
A sterilisation unit at the hospital is also being upgraded.
When the problem first appeared in June, an investigation concluded the water marks and white powder had been caused by water hardness and disposable blue paper lining used in theatre trays had created the sticky deposits.
The health board said patient safety had never been compromised at any time.
This time, black deposits have appeared on instrument trays, which is also related to water.
The hospital has been doing work on its usual steriliser and a back-up was being used, which was when the concerns started to be raised.
Cardiff and Vale Community Health Council (CHC) has received complaints from families who say surgery has been delayed.
One woman claimed her father had had a heart bypass cancelled three times in the past week, once in theatre, because the surgeon had not been happy with the instruments available.
The operation went ahead at the fourth attempt.
Phillip Williams, chair of Cardiff and Vale CHC , said: "You don't want to be sat in a bed in the cardiac unit for six to eight weeks waiting for an operation and then finding that it's been cancelled while you're waiting to go down to theatre.
"That for anyone is just not acceptable.
"I've had reports from about two or three people. I can't give you numbers but there's been a good number over the past six to eight weeks for people waiting for the operation."
However he added the health board had been very upfront about the problems and had kept the CHC fully informed on what was happening.
"In all fairness, they have responded by buying the new instruments," he said.
Dr Graham Shortland, executive medical director at Cardiff and Vale UHB, told BBC Wales: "The issue of the surgical trays has been given the highest priority.
"Each time it has invovled senior clinicians, managers and executives looking at the problem and trying to resolve the problem.
"We have been very careful in making sure that quality and safety are of the highest agenda and that in terms of delaying operations we have tried to reduce that to the minimum."
He added the issue had been largely resolved.
The health board said chief executive Jan Williams had been visiting surgical wards and speaking directly with patients to reassure them everything possible was being done.
Ms Williams said: "Staff on the front line are working incredibly hard to make that happen and are being backed by the support and investment of the health board with more than £200,000 being put towards new equipment and an upgrade to the sterilisation unit.
"The health board's handling of the challenges has been described as exemplary by Mike Simmons, the director of health protection for Public Health Wales.
"We will continue to make sure that the highest possible standards are met in terms of our care and the equipment we use. This does mean that there are occasions when clinicians have to prioritise surgery and that can lead to delays for some.
"I understand the anxiety and frustration that such delays can cause and I would like to assure everyone that we make every effort to avoid cancellations.
"The whole cardiac team is focused on making sure that patient safety and care is paramount. We apologise to those families whose members have faced delays but look for their understanding over the fact that the UHB has put patients first and will continue to do so."