The "significant factual error" appeared in Scottish Government accounts but was the fault of the UK Government Actuary's Department (Gad), according to John Swinney.
The position was clarified in letters to two Holyrood committees.
"It is clearly regrettable that an error of this type can occur and be identified only after scheme accounts have been published," Mr Swinney wrote.
"I am however able to assure the committee that this error has had no practicable implications for the Scottish Government's financial management of the scheme."
The error was in the 2011-12 accounts of the Scottish Teachers' Superannuation Scheme (STSS), laid before parliament in December.
It was identified in January following work to provide the Treasury with other details.
Mr Swinney said Gad overstated by £1.2 billion its estimate of scheme liabilities, later revised up to £1.8 billion.
It had no impact on the wider Scottish consolidated accounts, amounts paid to STSS pensioners or contribution rates paid by members of employers, he explained in letters to the Finance Committee and the Public Audit Committee.