Well, the company I work for we actually got into problems because of a badly formatted spreadsheet that one manager was using to re-order items for stocking, based on sales and inventory. We ended up with a lot of excess inventory and it took us 6 months to get on the other side of that issue. The guy had the common sense to quit before it was too late..

I have nightmare about errors in Excel, and when you wake up in the morning and your first thought is VLOOKUP , well it's time to take a break.

Personally, when I "inherit" a spreadsheet, I save a copy of it and look at it very carefully, formulas, vlookups, data formatting, hidden columns are so dangerous.. data formatting, change the font color for the entire thing to black, because i had seen people trying to hide stuff changing the font color.. I also look to unhide all the sheet and figure out how they correlate to each other.

On a day by day basis, i have a sheet where I do data cross checking, using conditional formating and logical formulas. if at the end of the report, all the check-marks are green, it means all is correct.

Also a big issue with me is a manager that doesn't really understand excel, yet he thinks he does, and that is something i need to lookout for.