At one stage, when we were selling a very expensive version of our spreadsheet product, we'd offer a pre-sales service of going through some of a potential clie

At one stage, when we were selling a very expensive version of our spreadsheet product, we'd offer a pre-sales service of going through some of a potential client's spreadsheets and converting them to our own system -- the aim, of course, being to show how much better our product was. In one case, a company was using a specific metric to drive their strategy; it was calculated as the difference between the sum of two columns. Kind of, "if the metric is positive, buy, if it's negative, sell". (I'm simplifying, of course.) Each column was populated with a recurring formula, for example lets say that column A had something like "=D3+E3" in row 3 and column B had "=D3-F3" in the same row, filled down the whole sheet. We factored out the repeating patterns, put them into user-defined functions, and found that we got different results to the original sheet. After spending a while trying to work out what we'd got wrong, we double-checked the original. Halfway down column B it switched from using its own formula to using a mis-copied version of column A's. So half the numbers in column B were wrong. In effect, the metric that the business had been running on for several months had been meaningless. The company never bought our software, but that may have been because they disappeared a month or so later.