A school in County Londonderry is costing the department of education an extra £3m after going ahead with a bigger building than approved.
The figure was revealed in an assembly report into improper spending and suspected fraud.
In the case of Magherafelt High school, the capacity of the school was changed from 500 to 600 pupils.
The report showed this was due to a lack of awareness rather than a deliberate attempt to avoid approval.
The original school was to have cost just over £8m, but the one currently being built is costing the department £3m more.
The report also contained allegations that second hand heating equipment was installed at La Salle High school in Downpatrick.
The suspected fraud is still being investigated by the police and there may be legal action.
Education Minister Caitriona Ruane has carried out an investigation and said there was no evidence of any other cases of suspected fraud.
However the minister said she has found significant weaknesses in the south eastern board's governance and internal controls.
More checks and monitoring are to be introduced.