Worked in banking IT when the first Excel virus hit...
That was a real wake-up call.
90% of the money went through Excel (having moved from Lotus 123, not long before).
First virus that hit us at the time randomly swapped 6 & 9 around in some cells of a spreadsheet.
When £9,6123,457 suddenly becomes £6,9123,457 people tend to notice, but with a spreadsheet full of tabs and each tab full of money and percentages, nobody could be sure we caught them all.