Note from CISCO - Defense against ATM robbery‏

A USEFUL TIP DO TAKE NOTE

PIN NUMBER REVERSAL

If you should ever be forced by a robber to withdraw money from
an ATM machine, you can notify the police by entering your Pin
# in reverse.

For example if your pin number is 1234 then you would put in
4321.
The ATM recognizes that your pin number is backwards from the
ATM card you placed in the machine.

The machine will still give you the money you requested, but
unknown to the robber, the police will be immediately
dispatched to help you.

This information was recently broadcasted on TV and it states
that it is seldom used because people don't know it exists.