More than £46m of RHI money has been paid out for boilers which did not have proper financial approval.
The 788 wood pellet boilers, which continue to receive money under the disastrous scheme, were accredited by the Department for the Economy between April 1 and October 29, 2015.
During that period the RHI (Renewable Heat Incentive) scheme was still open for applications despite it not having approval from the Department of Finance.
RHI inquiry chair Sir Patrick Coghlin will deliver his final report into the scheme on Friday.
According to the Department for the Economy, £46.7m has been paid out for these boilers between October 2015 and March 2019. Auditors from energy watchdog Ofgem have deemed this "irregular" spending, which is expected to hit a total of £77m by the year 2035/36.
In 2015, there was a massive spike in applications to the scheme which the RHI inquiry was told was partly due to information being shared with the renewable energy industry.
The then Department for Enterprise discussed possible tariff cuts to the scheme with firms in the biomass sector in the summer of that year.
One civil servant told the inquiry panel he thought there should have been less promotion of the scheme as officials already knew the cost for that year was twice the available budget. Ex-DUP MLA Jonathan Bell, then the minister responsible for RHI, told the inquiry that officials had not informed him of the spike in applications.
It was told that one adviser said "we could fill our boots" as the scheme was being paid for from the Westminster budget, rather than Stormont's.
The forecast cost of the RHI scheme in 2019/20 is £6-7m, thanks to cost controls which were put in place in 2017. Before these changes were introduced the total cost was expected to be around £1.15bn, more than twice the scheme's estimated cost of £405m when it was set up in 2012.
Those who may face criticism in Sir Patrick's report this week have already been given the opportunity to respond to his findings.
The Monday following his publication, Sinn Fein Finance Minister Conor Murphy will face questions from MLAs in the Assembly.
In total, the Executive's legal bill for the inquiry is estimated at £5.5m, with the overall cost of the nearly year-long investigation put at £6.7m.
The RHI scheme was launched by then economy minister Arlene Foster in 2012 and was meant to encourage businesses to switch from fossil fuels to renewable sources.
But the low cost of the wood pellet fuel compared to the generous subsidies created a "burn to earn" scenario where boilers could be operated purely to make a profit from the scheme.
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