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And here is what Crimestoppers has to say on this:
There is a common online scam advising people to insert their PIN number in reverse order to alert police if they are being threatened to withdraw cash.
This is a hoax and falsified information and we ask you not to act upon this.
If you do find yourself being threatened in this manner, we strongly recommend you contact the police.
And in case you were in any further doubt, here is a giant Snopes debunk.
Where you can see that this myth has been going around since September 2006 at least, confirms that ATMs have never had this technology nor are they likely to, and concludes with the rather killer line: "As Chuck Stones of the Kansas Bankers Association said in 2004: 'I'm not sure anyone here could remember their PIN numbers backward with a gun to their head.'"