In March 2017 there was a lot of media coverage about a dollar coin in your pocket that could be worth $1000. That coin is the year 2000 $1/10c mule, struck on the heads side with a 10c coin die and the normal kangaroo reverse (tails die). It can be tricky telling if a dollar coin is a mule error or not, so the folks at the Australian Coin Collecting blog wrote an article on the topic:
How to Spot a Real Year 2000 $1 / 10c Mule