I know of a company who has an internal policy of flat-out not paying invoices once they're more than X days old. Surprise surprise, they're a private equity firm!
Made a concession and offered Net60 to a F100 company. We don't usually do that, because it's weird to finance the balance sheet of a company with billions of liquid cash on-hand. But they insisted, so what the hell.
Now 30+ days late on Net60 and emails to them disappear into