Purchased our new house about a month ago and went and tested all the outlets in the house. Everything I found tested just right except for a single GFCI in the kitchen. When I pushed the test GFCI on the tester the outlet buzzed and popped. I assumed it was a bad outlet so I got a new one and went to replace it today.

I turned off the breaker to the outlet, validating the lead hot wire with a non contact tester and started switching wires. I did notice in directions it's noted to have no more than 6 black or white wires combined but I figured I would plug the new outlet in same slots as the old outlet was plugged in and see if it worked. I learned the hard way that the one neutral wire circled was still live by getting a nice shock as I touched it to pull it out.that the one neutral wire circled was still live by getting a nice shock as I touched it to pull it out. So I stopped and sure enough the no contact tester beeped even though the breaker for the outlet was off. Second sign something is off.

Went and turned off THAT breaker to make sure nothing weird happened until I could figure out what to do.

So what's going on exactly? It seems incorrect to have a wire that is located on a different breaker in this outlet, right? Or can I just plug it in where it was supposed to be and they go to two different breakers? I also noted the fridge went off on a different outlet after messing with this outlet when it wasn't off before. Though the non contact tester beeped saying it was getting power, a night light plugged into the outlet was very faint so maybe just a trickle of power with the breaker off but that one neutral on?

For information the house is really old (built in 1899) but the owners remodeled extensively about 12 years ago. I believe they intended to get everything done right and paid for good contractors but I'm wondering if something was just missed? I'm also located in upstate New York if that matters for code questions.

Thanks!