David Lyford-Smith is an expert at solving spreadsheet mysteries. Once, in a previous job, he was sent a payroll form to look over for a new starter. It had the number 40,335 in a random box, and payroll wasn’t clear why it was there. “So they assumed it was a joining bonus for the employee and drew up a draft pay slip with a £40,335 bonus,” he says. But, when it comes to spreadsheets, assumptions can be costly.