As said above, the wires in some electrical boxes may be controlled by more than one circuit breaker!
And because these situations exist, it is best to turn off ALL the power to the house before working on electrical wiring.
In the case of a MWBC, this would be two hots and one neutral. You could turn off power to one of these hots, yet still be shocked by the "shared neutral" or other hot wire!
More about multiwire branch circuits (MWBC)...
http://www.google.com/search?hl=en&...=0CAYQBSgA&q=multiwire+branch+circuit&spell=1
And because these situations exist, it is best to turn off ALL the power to the house before working on electrical wiring.
In the case of a MWBC, this would be two hots and one neutral. You could turn off power to one of these hots, yet still be shocked by the "shared neutral" or other hot wire!
More about multiwire branch circuits (MWBC)...
http://www.google.com/search?hl=en&...=0CAYQBSgA&q=multiwire+branch+circuit&spell=1