Judging from current eBay/Facebook usage here in Australia, one might believe that "mule" meant "mint error". But a "mule" a very specific kind of mint error. Specifically, a "mule" is where two dies that "belong" on two different coins, were improperly paired up and mashed together to make one coin. There are mules in many different coinage series worldwide; one of the most famous in the British Commonwealth series is the 1967 New Zealand 2 cents / Bahamas 5 cents mule. America has recently seen several "mules" where a die from a $1 coin is used to make quarters.
In the Australian series, prior to 2000 the most famous mule was the extremely scarce 1916 mule halfpenny, with the obverse from an Indian quarter-anna. But in 2000, the Mint made a new mule error when they accidentally used 10 cent obverse dies to strike some $1 coins. The 10 cent piece is smaller than the $1 coin, so this particular mis-match inevitably creates a noticeably much wider rim around the obverse. This "wide rim" is quite distinctive of the mule in question. If a 2000 $1 coin doesn't have that wide rim on the obverse, then it might be some other kind of mint error, but it's not a mule.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis