Found out today that with this plugin enabled on my server - no other plugins handling vending machines, or npc machines at all, when you purchase something excessively the currency used to purchase begins to spit back out of the machine.

Unloaded the plugin and the issue stopped immediately.

I've done some testinga and can confirm how to replicate this:

Go to an outpost vending machine, give yourself a bunch of currency for the item(s)

Buy out the order, and continue spamming as it refreshes.  Eventually it'll start spitting out items used as currency.