My colleague who dabbled in economics when deciding how many lunch vouchers to buy had a number of other money-related quirks.
One of the ones that I remember is that when paying for a purchase, my colleague would double the balance and give the cashier that much money. For example, if the total was $5.20, my colleague would hand over $10.40.
Just to see if the cashier reacted when pressing the Enter code appeared to have no effect.
Total is $5.20.
Cash tendered is $10.40.
Change is $5.20.
Most of the time, the cashier wouldn't pay any attention. Heck, the cashier wouldn't even question why my colleague handed over such a strange amount of money.
Sometimes my colleague would mix it up and instead add $6.66 to the total. For example, if the total was $5.20, my colleague would hand over $11.86, just to see the cashier's reaction when the cash register indicated that the change due was $6.66.
And then one day, magic happened: The total was $6.66. Without skipping a beat, my colleague handed over $13.32.