Speaking as an accountant...please dont write invoices with Vim, Excel or minesweeper. Just use a standardised package. The amount of botched homecooked invoices I've encountered is frightening & worse the people producing them seem entirely oblivious to the fact that they're wrong (sometimes wrong to the point of constituting tax evasion).
It really one of those cases of "just because you can doesn't mean you should"
That would be a country specific thing & is a bit of an outlier. As a simplified example though - sometimes the sales tax / VAT is calculated as 10/110 instead of 10/100 (or vice versa depending on law). Or there is some small piece of info missing that technically means the invoice isn't a legal tax invoice which means whoever you issued it to claims tax on that illegally (again country specific).
It's unfortunately a case of people dont know how much they don't know.
It really one of those cases of "just because you can doesn't mean you should"