From the interesting submitted article, "After subtracting the old rate from the new rate, the spreadsheet divided by their sum instead of their average, as the modeler had intended. This error likely had the effect of muting volatility by a factor of two and of lowering the VaR . . ."
This astounding story is a really nasty example of why "always comment your code" is a good idea--but only if someone else with knowledge of business logic reviews the code.
But this is really the nub of the problem. By default (and most of the time), Excel hides the formulae and comments - requiring clicking into cells to figure out what's going on. By only exposing a glimpse of the whole model at a time, it's very difficult to get an overview of how the model interacts.