Counterfeit
For instance a counterfeit bank note will look enough like a bank note that, apart from careful inspection, people will think it is legal tender to the extent where it can be used as real money.
Because of this ability to literally print money if you get it right, counterfeiting is a problem for all major currencies and when surveys are done, for instance by the UK royal mint, the amount of fake notes and coins in circulation comes back surprisingly high - it may be that around 2% of £1 coins in the UK are actually fake for instance.
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