There was a HN comment about competitors tracking how many new signups are happening and increasing the discounts/sales push based on that. Something like this.
In a business I once worked for, one of the users of the online ordering system represented over 50% of the business' income, something you wouldn't necessarily want them to know.
However, because the online ordering system assigned order numbers sequentially, it would have been trivial for that company to determine how important their business was.
For example, over the course of a month, they could order something at the start of the month and something at the end of the month. That would give them the total number of orders in that period. They already know how many orders they have placed during the month, so company_orders / total_orders = percentage_of_business
It doesn't even have to be accurate, just an approximation. I don't know if they figured out that they could do that but it wouldn't surprise me if they had.
This is also something that depends heavily on regulations. In my home country, invoice numbers have to be sequential by law, although you can restart the numbering every year.
Yes, even if it's not a legal requirement it's definitely best practice to have sequential invoice numbers. I thought about this at the time but these numbers aren't invoice numbers, only order numbers.
A sequence per "series", where a series can be a fiscal year, a department or category, etc. But I am not sure if you can have one series per customer, I only find conflicting information.
You can have more details here, in the section "Complete invoice":
That's happening everywhere. You can order industrial parts from a Fortune 500 and check some of the numbers on it too, if they're not careful about it.
