"911" is the emergency number for countries like the United States and the United Kingdom. And, saying "nine-eleven" or "9/11", which is the same as saying nine-one-one or 911, will cause Siri to dial the emergency number: "911".
Siri is an intelligent personal assistant on the Apple iPhone, iPad, and iPod that lets you use your voice to send messages, make calls, set reminders, and more.
The Siri Practical Joke
Tell Siri ” Nine eleven” her response is awesome
or
Tell Siri nine eleven, it’s crazy what she says back
The practical joke almost got me too, because when I saw the message, the first thing that came to my mind was September 11, which is the day terrorists used planes to attack the United States killing hundreds and destroying the World Trade Center buildings in New York City.
The joke is spreading on social networking websites like wildfire, so sooner or later, may end up on your Facebook Wall/Timeline or Twitter feed. So, please do not say: "Nine Eleven" ("9/11") to Siri, because she will dial the emergency number: 911.
Imagine the stress on the 911 emergency operators who will have to answer all of those accidental calls, and even call back most of the numbers because most people will hang up after they have realized their mistakes, which will cause the emergency operators to call back the number to ensure that they are not missing actual emergencies.
And using or tying up all the 911 emergency resources, can be detrimental, because people who are calling about real emergencies will not be able to get through to the operator on time.
So, please do not ask Siri about "9/11" for the sake of the 911 emergency operators and the public that depends on them.