Here in entry 594 I wondered how $0.18 was 10% of $1.50, but I looked at the receipt again.
It's not 10% of $1.50. It's 10% of $0.25 six times.
Why does that matter?
Because the cash register rounds fractional pennies up. 10% of 25 is 2.5, and the cash register rounds it up to 3.0...and 6 times $0.03 is $0.18, which is actually 12% of $1.50.
I always assumed that the thing took off 10% of the pre-tax total, not 10% off of each unit. That's pretty cool. But even so you can really only game the system when it comes to buying a number of items with a two-digit price, and it really only works well if the ones digit is five or less.
Another thing most people wouldn't care about, and wouldn't bother to figure out if they did.