[darkr]

As a tool for rough approximations, spreadsheets are amazing. Beyond that though:

* Lack of unit (or any) testing

* Lack of good versioning practices and code review (e.g emailing around a sheet that's been doing the rounds for 15 years in various different guises and formats and has who knows what horrors lurking)

* Lack of typing (e.g doing a SUM across a dataset consisting of "3", $3, 3, "III", an emoji of the number 3, a jpeg screenshot of the number 3 - might not add up to 18)

* Lack of precision (rounding errors)