The hard to make out coin is Hong Kong's one dollar coin (about 13 cents). It is pretty similar to Taiwan's ten dollar coin (about 34 cents). Recently a tricky cashier gave me the Hong Kong coin instead of the Taiwan coin.
As a foreigner you always have to be careful. If a worker accidentally accepts a fake coin or bill, they will usually try to give it as change to people who they think are unsuspecting, i.e. foreigners. To my knowledge this is the first time it's ever happened to me, and at the time something seem strange about the coin but i was in a rush and didn't really pay attention.
Thankfully it's not so common, whenever you give money to a worker, they will usually inspect it very carefully. A few years back a foreigner friend was given a fake 500 dollar bill (about 17 dollars), and after a few attempts was able to actually spend it. But now a days I don't think even this coin would be accepted anywhere. But the fact that somebody was tricked and then i was, is a good reminder to always pay attention.
