@MJG017 there are heaps of different things to look for in coins. And the lower the mintage of a coin, the more money you can get for it. Especially if it is good condition.
I would love to find these error coins.

1966 20c Wavy Baseline
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The 1966 “wavy baseline” 20 cent coin, for example, has been known to fetch up between $3,000 and $5,000 when in perfect condition.


The "5 cent flamingo error" is an Australian 5 cent coin from the year 2001 (and to a lesser extent, 2002, 2003, and 2005) that features an extra piece of metal on the number 5 on the reverse side. This additional metal, caused by a die crack, resembles a flamingo's beak when the coin is turned upside down, hence the name "flamingo error". This die variety can be worth a significant amount above its face value, with recent sales ranging from approximately $37.80 to $70.

The Australian 2016 "Alien" 5 cent coin, caused by a die clash, is a rare error coin valued in the hundreds to potentially thousands of dollars, depending on its condition and grade. Uncirculated examples can command higher prices, with some sources suggesting up to $4,000 for uncirculated specimens, while circulated coins are worth less, possibly around $600 or a few hundred dollars.