Post Office bosses issued a John Knox stamp bearing the slogan "Reformation 1559".
But last night, historians slammed the blunder as the Reformation happened a year later in 1560 - with this year marking the 450th anniversary.
The mistake was spotted by a 75-year-old philatelist from Aberdeenshire who - unfortunately for the Royal Mail - also holds a PHD in church theology.
Retired personnel manager Dr Sandy Waugh, from Banchory, said: "They have got the wrong date. It is a bit off and very embarrassing
"They should have done more research.
"After all, it's the 450th anniversary of the Reformation, which banned Papal supremacy - and to compound the timing of the mistake even more, the Pope is due here in September.
"This must be up there with their biggest mistakes. I think it was nice to have a stamp of John Knox but they could have at least got the date right."
The Reformation established Scotland as a Protestant country and broke ties with Rome and the Pope.
Knox led the revolution that sparked a turbulent period across the country and shaped Scotland for centuries to come.
The Royal Mail's mistake has also reignited a row about the failure to recognise this year's anniversary.
Professor Tom Devine, from Edinburgh University, has branded the low-key approach adopted by the Scottish government as "scandalous".
The historian said: "The Royal Mail has got the date wrong. The formal date of the Scottish Reformation is the middle of 1560, when the act of parliament that banned the Mass and cut the link with Rome was passed.
"But at least the Royal Mail have recognised the momentous event, which is more than the Scottish government, and to some extent the Church of Scotland, have. It has all been very low key."
The Royal Mail issued the stamps as part of a series about the reign of the Stewarts.
A Royal Mail spokeswoman said: "The stamp featuring John Knox is specifically designed to commemorate the role played by John Knox in the Scottish Reformation.
"It focuses on the importance of the year 1559 in John Knox's career, as that was when he returned from exile in Geneva to lead the campaign for the Scottish Reformation.
"The significance of the year 1559 is explained in the presentation pack published by Royal Mail, of which the John Knox stamp is one part.
"The stamp is about John Knox and, therefore, there has been no error made by Royal Mail on this stamp."
In August, First Minister Alex Salmond said that requests to the government for support in marking the 450th anniversary of the Reformation in Scotland would be considered.
A government spokesman said: "We are working with the Church of Scotland and others to ensure this important anniversary is properly commemorated, and we will make further announcements in due course."SIC: DRCUK