2019 Dollar Discovery
In mid-October 2018 Australian coin collectors began to notice 2019-dated $1 coins in their change: besides the unexpected year (future-dated coins almost never appear in circulation), the standard Mob of Roos reverse also bore an extra '35' between two of the kangaroos and a privy-mark of one of the letters A, U and S. There was early speculation that the Royal Australian Mint had accidentally released some non-circulating collector coins into circulation both due to the early appearance of 2019-dated coins and the fact that privy-marks had only been seen on non-circulating coins until that point. The Royal Australian Mint were quick to affirm that the release of the coins was no accident and that their purpose would be revealed soon (Chung, https://finance.nine.com.au/2018/10/16/14/23/time-travelling-coin).
On November 1st 2018 the Royal Australian Mint revealed that the one million of each of the privy-marked coins had been released as part of a competition in the lead-up to the 35th anniversary of Australia’s $1 coin. To encourage coin collecting, people who found each of the different privy-marked coins in their change were eligible to enter the Dollar Discovery competition at https://dollardiscovery.com.au/ with eight major prizes of a tour of the Royal Australian Mint and a 1 kilogram silver coin.
A winner in each state and territory of Australia was announced on May 14th 2019 (https://www.ramint.gov.au/publications/winners-royal-australian-mint-nationwide-dollar-discovery-announced-0).