In addition to formulas not being replicated properly, another problem is when a cell that appears to contain a numeric value is treated as text by Excel, because it contains an embedded space or something. That's easy to overlook when working with a long series of numers. So when applying a @SUM formula across the column, or just highlighting a range of cells, the malformed "number" is excluded from the total.

10.5

12 .1

4.5

-------

15.0

Here is an actual example where the mistake ended up in a published economic report that was cited during the 2012 presidential race.

http://www.businessweek.com/articles/2013-04-18/economists-spreadsheet-error-upends-the-debt-debate