Just to expand on that - many banks employ Excel basically as a frontend for their own analytics/database (using own functions with addins). A popular trick is to allow functions to return handles that stand in for objects. Using that, you can run not only Monte Carlo simulations and PDE solvers in a pricing sheet, but even run a whole trading book including risk in Excel.

Of course, you can then also overwrite certain cells to pad your PnL, as a few Credit Suisse traders did in 2007/2008 to hide their losses in mortgage backed securities. They basically overwrote bond prices to some fictitious value, overstating the PnL of their position by $540m or more...