In which cases would this be a German tank problem for you? I can think of apps where you want investors or something, but I can't help but wonder if they'd be savvy enough to check for that.

I do know that I try to make my invoices to clients a bit more impressive, because I don't send many of them and they most definitely do notice if they get invoice #3 in May. Main problem is that in my country the rules for invoices are both murky and stringent, so I'm pretty much limited to a <year>0000<invoice no.> format.

Well if you have an 'order #', your competition can make an order a week and see how many other orders you've gotten in the meantime.

> in my country the rules for invoices are both murky and stringent,

I sympathize, as the rules were like that in Italy, where I lived for a long time.

You can use hashids for this problem [0] This just "encrypts" the primary key with a given salt.

[0] http://hashids.org/