Fake Caller ID.

Fake caller ID (making any number appear on a Caller ID system) has been possible for quite a long time but the phenomenon seems to have taken alarming proportions thanks to technological improvement - it has become easier and more available.
Today there are many websites providing Caller ID spoofing services, and there is no need for any special hardware. Some even provide optional voice scrambling, to make the caller sound like someone of the opposite sex.
While the notion can be a bit scary, esepcially if you get anonymous calls, there are also some funny but chilling stories like these two found in USA Today:

In a typical case, someone will be jarred in the middle of the night by repeated telemarketing calls. He checks Caller ID, calls the number — which is false — and starts "cussing out" the person at the other end of the line. And that poor guy was asleep. It wasn't him at all.

or this one

In one case, SWAT teams surrounded a building in New Brunswick, N.J., last year after police received a call from a woman who said she was being held hostage in an apartment. Caller ID was spoofed to appear to come from the apartment.

While you can imagine all sorts of possible pranks, or even annoying intrusive telemarketing use of fake caller ID, you can also esasily see its potential use for fraud, even with possible deadly