Is it possible for an external device to burn out the USB port?

USB slippers? Surely you're kidding.

OK, so maybe they actually exist (because for every dumb idea there's somebody eager to make a buck selling it). But if so, do your computer a favor and don't use it to heat your toes. Go get yourself a little USB AC adapter/charger block for an iPad and pug your slippers into the wall through that. Anyplace that sells iPads will have them. Get your computer out of the warm-feet business.

The USB ports in your MBP are protected against over-current. If you draw too much, the port shuts down and tells you so. I don't think you can burn the ports up for that reason, but a product as crackbrained as USB slippers might be the exception if there is one.

...But I once connected it and a mouse to a USB hub, then to a single MBP USB port, and the mouse stopped working in a few seconds, and came back normal only after I switched it to another port....

Likely the hub is only barely able to handle the 500mA requirement. Many powered USB hubs have "cheap" power adapters and can't handle any high surge situations. Many devices like heating elements and motors have "surge currents" where the initial powering on of the device has a much larger current flow than during normal usage. Think of it like riding a bicycle. You have to push hard on the pedals to start off, but it's easier to pedal after the bike is moving.

...Now the slipper is connected to a port by itself...

If you mean the device is plugged into the Mac's USB port, then you should know that all Macs for the past few years provide more than the 500mA minimum for USB. Some other computers also provide higher power. But most USB hubs I've seen were limited to 500mA.

If you are plugged into the hub, not the Mac, then the power adapter for your hub may be barely adequate or possibly even defective.

...but I am just afraid that the USB slipper can go malfunctioning all of a sudden and drain a lot of current to burn the USB port.

IMHO, I would stop using the heater with any computer USB port. There is always the possibility that eventually something could short out in the heating element and cause a catestrophic surge through the USB port. The electronics in computers are quite delicate and I'd be paranoid about the device damaging my computer. (BTW, I have an electrical engineering background so I'm paranoid about power issues all the time anyway. 😝 ) Even if the heater is plugged into a hub, the surge current could damage the other devices plugged into the hub. While not as catastrophic as a dead computer, I'd still be irked to have to replace my mouse, digital camera, printer, webcam, etc.

Instead, I recommend plugging the heater into it's own "power adapter." One of those that give a USB port to charge a cellphone or whatever. Plug the power adapter into a power strip. Place the power strip in the floor next to the slippers. That way you can conviniently turn the strip on and off with your toes as you're putting on your slippers. 😎

...(because for every dumb idea there's somebody eager to make a buck selling it)....

You mean like this USB Pet Rock? It uses zero amps so is "energy efficient"...and we don't have to worry about blowing out the USB ports! 😁

...That's why I thought 500mA was the maxmimum current. So I was wrong about this?...

The specifications for USB in general says the port must supply at least 500mA. The computer manufacuter can supply more, but they must provide at least 500mA because manufactuers of USB devices will assume then can get at least 500mA. Although Apple and others supply more, there is no guarantee above 500mA.

...Oh by the way I should correct my previous comment about the USB heater slippers being smart or not. I love the idea about heater slippers (they are really efficient!), but I don't like the USB part.

I have no problem with the heater. (I used to live in Arizona where winter temps in the desert gets down to the 30s at night.) It's using the computer's USB port that is the issue. While I understand the desire to be "energy efficient" and so use the readily available computer USB ports (since they're already on,) it's the risk of frying the two thousand dollar computer for a ten dollar gadget that makes me paranoid. 😮

Recently my local Costco started selling these Tripplite UPS' that have a USB power port built-in. So I've been using that to charge my Bluetooth headset and such so that I don't have to remember to unplug all those power adapters to reduce phantom power usage.