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.