Many phones in modern offices have a button marked simply with the word "Feature."

With a little bit of research and curiosity, you can harness the awesome power of this obtuse little button.

For example, one slow day at work, I pulled out a Quick Reference card attatched to the bottom of the phone. I then learned how I could reroute other people's phones all around the office with only a few easy key strokes. So, if you ever become a disgruntled employee, you could engage in some super inter-office phreaking all while looking like your busy at work checking your voice mail or following up with a client. Try these codes to get you started:
FEATURE + 32 + ext. (Reroute incoming calls to this phone to ext.)
FEATURE + 33 + ext. (Reroute incoming calls to ext. to this phone)
FEATURE + * + 33 + * (cancel all reroutings involving this phone. Cover your tracks)

(Disclaimer: I don't know if these codes are standard for most systems, or even if I remembered them correctly. Self experimentation is encouraged. If anyone has more information, feel free to include it below).