The new £1 coin was supposed to be the most secure in the world, but after less than a month some are apparently losing their middle part.
The 12-sided coin, which features a gold-coloured outer ring and a silver-coloured centre, was rolled out in March.
But problems are already being reported, such as warped coins, gaps between the metal parts, cracks and blemishes.
Some have entirely lost their silver centre, which features the Queen's head, according to the Sun.
Hopeful punters are even auctioning them online as a collector's item.
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One shopper who was handed a misshapen coin with a gap between its outer ring and centre core said she was baffled at how it slipped through.
The Royal Mint says a few defective coins were always likely to slip past its quality control due to the sheer volume being made.
"The Royal Mint produces around five billion coins each year, and will be striking 1.5 billion new £1 coins in total," said a spokeswoman.
"As you would expect, we have tight quality controls in place. However variances will always occur in a small number of coins, particularly in the striking process, due to the high volumes and speed of production."
Authorities described the new coin as the most secure in the world, with features that would make forgery very hard.
The coin features micro lettering and milled edges - and even a hologram that makes a "£" sign change into the number "1" when viewed from different directions.
But not all has gone according to plan, and the launch was hampered by concerns that the coins would be incompatible with shopping trolleys and vending machines.
The old coin and the new coin will both be in circulation for about six months, until the round pound ceases to be legal tender on 15 October.