Re: Outlet on Two Breakers?
It's very possible that circuit you shut off at the breaker is part of a Multi Wire Branch Circuit ( MWBC ) which is simply two circuits being fed by individual hot conductors but sharing the same neutral. This can be a dangerous arrangement since the neutral is returning current from the other circuit it shares.
This is why up here in Canada it's mandatory that all MWBC hot conductors be connected to a double pole common trip breaker -- in your case in the States two single pole breakers connected together with handle ties.
This way when you want to shut off the circuit you are working on both are disconnected to remove any return current on the shared neutral.
"" an ounce of perception -- a pound of obscure "