Why did my vacuum plug just melt inside the wall outlet?

That massive pop was a dead short-circuit caused by a high-resistance connection that finally failed. When your housemates yank the vacuum by the cord, it pulls the internal copper wires away from the plug's brass prongs. Eventually, the connection becomes loose and generates extreme heat, or the wires fray enugh to touch each other. That heat is what melted the prong off and scorched your outlet. You need to find your panel and flip the breaker for that room immediatly if you haven't already. Even with the vacuum unplugged, that scorched outlet is a major fire hazard because the internal contacts are likely compromised and coud potentially arc. Do not try to pull the prong out with pliers, even with the power off, because the heat has likely welded the metal to the internal guts of the receptacle. The outlet is completely ruined and needs to be replaced. Once an outlet has been 'charred' or 'carbonized,' the plastic becomes conductive, wich means it can keep smoldering or tracking electricity even if nothing is plugged into it. You're looking at a ten-dollar fix for a new heavy-duty receptacle, but you also have to cut the cord off that vacuum and install a new replacement plug end.

If you aren't comfortable opening the wall, call an electrician. They'll need to check the wires inside the junction box too. If the heat was intense enough to melt a prong, there's a good chance the insulation on the house wiring behind the outlet is crispy or melted. If those wires are damaged, they need to be trimmed back to clean copper before a new outlet is installed, otherwise, you're just begging for another electrical fire.

Your housemates are gonna burn the place down if they keep that up. I've seen so many outlets get loose becuase people yank cords at an angle, and that's usally when the arcing starts. Since you gotta swap the outlet anyway, grab a commercial grade one. They're only a few bucks more but the grip is way tighter than the cheapo residential ones. It won't stop your friends from being rough, but it'll handle the abuse way better. Just make sure the vacuum motor's still okay after that pop, sometimes the surge kicks back and kills the appliance too.

Seen this a thousand times back when everythign was built with those cheap 'speed-wire' push-in terminals. Those things were a death trap waiting to happen. If that outlet was back-stabbed, that heavy vacuum load probably finished off a connection that was already cooking for years. When you swap it out, don't use those little holes in the back; wrap your wires around the side screws the old-fashioned way and cinch 'em down tight. And while you're at it, check the wire insulation inside the box. If it’s that old rubber stuff, it’ll crumble like a cracker if it got too hot, and then you’ve got a real mess on your hands.

Whoa, that sounds terrifying. I don't know much about this stuff, but is the 'carbonized' plastic thing really that dangerous? Like, can the outlet actualy start a fire by itself even if I don't have anything plugged in anymore? I'm defintely too scared to touch the wires myself, but if I call an electrician, how do I know if they're actually checking the insulation deep in the wall or just swapping the plate? Also, what exactly is a 'commercial grade' outlet vs a normal one? Does it just look diferent or is it made of different metal inside?

Oh my god that is absolutely wild! I’m totally new here becuase I’m trying to learn how to fix stuff around my new apartment, but seeing that a prong can literally WELD itself inside the wall is making me want to go check every single outlet I own right now. On a related note, does the type of vacuum acutally matter for this? I just bought one of those super powerful shop-vacs for my garage and now I’m wondering if those heavy-duty motors pull way more power and make this kind of meltdown even more likely if the cord is loose?

Update: I killed the power and pulled the cover off to look closer. It definitly looks like those 'back-stabbed' connections mentioned. The wire inside is acutally fused to the terminal. Since I need to replace it with a commercial grade one, how much of the wire do I need to strip back? The copper looks a bit discolored, and I'm worried the heat damaged the insulation further up the line.