It isn’t clear how the strange quirk — which some seem to have been using as a prank — happens

Siri can be tricked into phoning the emergency services just by telling it to charge the phone.

Giving the iPhone personal assistant the instruction to “charge phone 100%” means that it automatically calls the police, the Verge has found. No further questions are asked, and users have five seconds to end the call.

It seems to happen because the phone is hearing the words as an instruction to “phone 100”, and it interprets 100 as the emergency services. Telling the phone to “charge phone 999” and “charge phone 911” have the same effect.

But it isn’t clear what the “charge” does to make the phone ring straight away. Asking Siri simply to “phone 100” opens up the phone app, to let people do it manually, so it appears that for an unknown reason the “charge” prompt makes the phone skip that part and go straight to the call.

Unlike other funny or charming Siri quirks — like its playing Rihanna when instructed to make fart noises, or its correction of the mis-gendering of Caitlyn Jenner — the effects of this one aren't so much funny as very dangerous.

As the Verge points out, there is little trail of the quirk being used online. But tweets going back to July show that some teens are using the message to prank their friends by tricking them into calling the police — which is obviously a stupid thing to do.

The Verge suggests that the feature could have been added as a subtle way to call the emergency services without bringing attention to having done so. But since the feature isn't advertised by Apple and doesn't seem like it would be stumbled on innocently, it's just as likely to be a bug with the software.