If you use iTerm
.. and a mac of course, since iTerm is a terminal app for osx, be weary of one shortcut.
apple-i
I'm sure it's really really useful when you want to use it. I'm sure it's the coolest functionality ever. When you want it. I'm just not sure it should be so easy to turn on by accident.
What does it do, you ask?
Send keyboard input to all open tabs. Oy.