On this day in 1979, a computer program called VisiCalc first shipped for the Apple II platform, marking the birth of the spreadsheet, a now-ubiquitous tool used to compile everything from grocery lists to Fortune 500 company accounts.

And that’s why October 17th is Spreadsheet Day, celebrated by fans of the form.

Happy #SpreadsheetDay - may your cells be plenty and your sheets prosper! #accounting pic.twitter.com/D1GSDtrOZN — Sage Accountants (@SageAccountants) October 17, 2014

It may seem weird to muster much enthusiasm for Microsoft Excel, the spreadsheet of choice these days. But it’s no exaggeration to say that spreadsheets—originally conceived by Harvard Business School student Dan Bricklin to crunch numbers for a class assignment—are inextricably woven into the fabric of modern commerce. From their earliest days, spreadsheets have been used for a dizzying array of tasks, as Bricklin told CFO magazine:

As I recall, among the first hundred users were people in the medical field, who used it to calculate factors pertaining to anesthesiology and open-heart surgery. When you find out something like that, you gulp, and hope your insurance agent doesn’t find out about it. It was also used to figure out the optimum organization of slot machines on the floor of a casino.

The spreadsheet’s flexibility is what makes it so useful—but also incredibly dangerous. And that makes Spreadsheet Day both a time to celebrate the tool’s awesome power as well as fear its capacity for destruction.

In recent times, simple spreadsheet errors have contributed to everything from misguided justifications for global economic austerity to billion-dollar bank trading scandals. These and many, many more spreadsheet snafus are documented in one cringe-worthy collection on the site of the European Spreadsheet Risks Interest Group, probably the forum that thinks the most deeply about the corporate world’s complicated relationship with Excel.

Don’t we all. (Amazon)

Over the years, researchers who peer into company spreadsheets decry a “very disturbing picture” of worksheets littered with mistakes. Academics at Dartmouth got access to live spreadsheets from a group of companies and found only a third were error-free. Many mistakes were minor, but some skewed numbers by as much as $100 million.