I was adding an outlet and removed a blank cover over an existing outlet to see what was behind it, and noticed it had a red wire that was capped, and a hot and neutral that were capped together under one nut. I put my multimeter to the hot/neutral combo and it sparked and burned my multi tip. I went and cut the main and uncapped it, and noticed that the end of the neutral was completely gone and burned on the end. There was no melted piece, so my guess is that someone (possibly me) twisted them together and then it melted through the neutral until the wire just evaporated? Interestingly, the inside of the plate had my handwriting on it and said "220v - Marked as Dining Room A/C in breaker. White is ground."
Maybe this used to be the wiring for my old A/C unit before I moved in 15 years ago and replaced my AC? I checked the breaker box and I have a separate breaker also listed as A/C and my AC works just fine with this breaker off. I measured the hot/neutral and it measures as 115v, not 220. The breaker is 30 amp. I restripped the wires to remove the burnt ends and then capped them individually and put them back in the receptacle. Everything seems to still be running fine in my house and my A/C works. I then cut the breaker to it so they aren't live.
I have not touched this outlet in at least ten years. I guess my question is, is that a danger to have the hot and neutral capped together and if so, why didn't it trip the breaker, which has apparently been on this entire time? And lastly, I assume it's fine now that I capped them all individually and cut the breaker?