Originally Posted by jbfan
The gfci was being fed by a muliti wire circuit. To prvent this from happening in the future, you will need to turn off the other breaker using that neutral.
A double pole breaker forces you to turn off the other breaker, and makes sure it is the correct one.
Most double pole breakers are fairly reasonably priced and many of us agree they are a good idea in a residential setting, as they ensure both legs of a MWBC get energized at the same time.
It might be worth it to have an electrician comes in for a few hours and identify what you have on MWBC's and install double pole breakers for them.