The 999 idea is good, but as for the other ones, oftentimes this data is recorded in, e.g., a spreadsheet, text file, or vanilla SQL table, so there's no way to

The 999 idea is good, but as for the other ones, oftentimes this data is recorded in, e.g., a spreadsheet, text file, or vanilla SQL table, so there's no way to throw an exception. You could store something like #N/A or NULL in the bad cells, but a lot of bad code and spreadsheet formulas will interpret that as 0 again, and then we're back where we started.