I once inherited a set of projects that were part of a company-wide programme; my bit was about £10m out of the £100m.
My team accountant was struggling with the budgets (a vast concoction of inherited and created sheets across multiple files), but there was an error he just couldn’t find.
So one weekend I took a copy and attemped to desk-check it (I was a Cobol programmer from the days when we wrote them onto coding sheets, rather than having terminals to input ourselves).
After about 10 hours I found the error, someone had added £179,000 onto the end of a 3-line formula…
Once I found that, I had him desk-check the lot – we were about 1/2 million out across the programme. It took a lot of hiding did that 😉