I remember hearing about a mythical spreadsheet floating around for modelling something to do with our national grid a few years back.

It would take about 12 hours to calculate, and would error out before finishing about 30% of the time. It needed to be run once a day for something reasonably important.

I don't use Excel much these days, but I do point people to a video if they do plan on doing anything:

* [You suck at excel - Joel Spolsky](https://m.youtube.com/watch?v=0nbkaYsR94c)