If you really want to hit your face on a desk, wait until you encounter a user who doesn't know how to tabulate or sum anything in Excel, and uses it as a big grid-based text editor, manually using a desk calculator to sum up columns, or perform various other formulas on some selection of cells.
also, prepare for the user to suffer some sort of existential crisis or breakdown, possible tantrum after you show them how to do these tasks automatically with a simple excel formula.