Mint errors occur because of unintentional problems while minting a coin (Some examples are clipped planchets, die grease, die rotation, off-center, double-struck, brockage, die cap, doubled die and etc.) or they are done purposefully by the mints that make them (like the 1913 V-nickel that is worth over a million dollars)
This is a 10 lira coin from 1987, made of aluminium. The first one is double struck and the second strike is 95% off-center.
The second one is also double struck but the second strike is 90% off-center. Slightly larger error coin.
This is a 1 lira coin from 1959, made of a steel alloy called "acmonital-52". Coins can be cancelled because they night be test strikes that got destroyed after or they had errors in the coin. (Like die wear or other errors like off-centers or clipped planchets)