A few weeks back (I wanted to mention it closer to when it happened but I was lazy with scanning of things) I went to get my usual Diet Dr. Pepper from the Pepsi machine in the break room/kitchenette at work. The thing costs $1.25 (down from several months and another vender back when it was $1.35). So I do my usual ritual of making sure the corners of the dollar are as unfolded as much as possible put it into the dollar feeder and then plunk my quarter into the coin slot. Well it did go plunk right into the coin return bin. The machine didn't take my quarter, so I put it in again. Plunk same thing it didn't like this quarter for some reason. I look at the quarter and don't see anything wrong with it just an ordinary quarter, so I fished out another quarter from my pocket and plunked that one in and the machine liked it fine and dandy and I finally got rewarded with my Diet Doctor Pepper. So the quarter?
This is it!
A nice 48 year old quarter from 1964 from before I was born. So why did the machine not like this quarter? Well it turns out that 1964 was the last year that the US Mint made quarters using approx. 90% Silver 10% Copper. Starting the next year 1965 they changed to 75% Copper and 25% Nickel. Modern vending machines are mostly calibrated to only accept the newer combination of metals. I didn't know that until searching on why my quarter wasn't accepted. I thought they just used weight/size to determine which coin, but apparently they have a tiny mineralagist in the machine with an electronic microscope testing the mineral integrity of each coin as it speeds down the slot.
I guess this is a good way to find older coins, or at least to test the age of the vending machine.