what could go wrong?

[usonian]

11. Reverse engineering old stuff is damn near impossible.

Even if you have access to things.
You can get a handyman book on wiring at any hardware store, and they will have sound advice, but if you see old stuff like this:

Call for a pro. Rip out and replace, or you won't have a home for very long.

Some years ago, I was walking to my front door and saw some flashing in the neighbor's garage. Oh, they're welding. I knocked on their door and they were all watching TV, definitely not welding. So, we called the fire department and deployed our garden hoses in the meanwhile.

Old electrical stuff.

I searched for free electrical wiring books and this page came up.
https://electrical-engineering-portal.com/download-center/books-and-guides/electrical-engineering/electrical-wiring
Has a download link to a Black and Decker book, which looks like the real deal.
Says it's compliant with the code up to 2017, which is better than what you have.
And I suspect it's not entirely Kosher to share this way.

But if it saves a life or two, or a home, good.

Just make sure that the work is up to code. It's there for your safety.

Qualifications: Physics major, and former Third Class Electrician's Mate!

What can go wrong?
• Ground/Neutral wire is hot (They didn't have a third grounding wire in those days)
• Something got crossed, so powering on blows a fuse or breaker.
• There are still old wires in the branch, and insulation frayed (or critters ate it)
• A connection is not made properly, and gets loose over time and sparks.
• Aluminum and copper wires are joined without a magic connector to join dissimilar metals.
• Wire nuts are not properly done. An electrician who put in my generator said that the code had changed in terms of pre-twisting the wires.
• New codes require metal shields to prevent screws and nails from penetrating wires in the walls.
• Stapling/clamping wires improperly (ouch)
• Not being able to tell "silver" from "gold" on the switches, etc.
• Inconsistent color coding of the wires. (black/white/green, usually)

Most likely that the neutral wire ends up being hot, or that wire nuts/joints are not done right and tight. I'd put in that grounding wire and test like crazy.

And do check in from time to time!!