Complaints about GPs and dentists rose by more than a third last year, according to revised official figures.
Statistics published in September initially indicated that grievances against these services had fallen by 15% between 2011/12 and 2012/13.
But a revision to these original figures shows that the number of complaints in fact rose by 36%.
The health information division ISD Scotland revised the number of complaints against family doctors and dentists from 2,992 to 4,804 for 2012/13, compared with 3,538 in 2011/12.
The organisation said a "formula error" had been discovered.
Conservative health spokesman Jackson Carlaw said: "Considering the Scottish Government unleashes all these health statistics at the same time, in the hope they won't be scrutinised as closely, the least we could expect is for them to be accurate.
"This isn't a small statistical error.
"It's a major oversight which created the impression that doctor surgeries and dentists were performing considerably better than last year.
"In fact, the opposite is true and the Scottish Government now needs to explain why there has been such a huge increase in complaints.
"It also has to offer a proper explanation of how such a sizeable mistake like this could have slipped through unnoticed until now."