After buying a new set of tires, a mechanic noted that he saw brake fluid dripping from one of the brake calipers, near where the hose connecting it to the rest of the car met the caliper. (The caliper is the hydraulic clamp that actually grabs the brake pads / rotor to stop the car.) I do all of the work on my car myself, so I bought a replacement hose and caliper. My car is pretty ancient (1986), so at the first sign of trouble I like to replace anything that could be a problem with new parts.
Unfortunately, the threaded pipe fitting where the hose connects to the metal pipe that goes to the rest of the car was rusted quite solidly. I tried to use a rust penetrating oil (parts blaster) and let it soak for a few days, but to no effect.
One trick that is used by mechanics to loosen rusty objects is to use thermal expansion to fracture the rust layer between them. The thermal expansion of the outer object (such as a nut) creates enough stress to break the rust, and the rust acts somewhat as an insulator slowing the rate of expansion of the inner object (such as a bolt). So, I heated the hose fitting with a torch in an effort to free it from the brake line. I figured that brakes themselves get hot enough from regular use that the fluid would not have a problem with some heat from the torch.
This was a horrible mistake.
I did succeed in getting the fitting hot enough that I was able to turn it. As soon it turned, boiling brake fluid gushed out of the fitting. Unfortunately the fitting cooled enough that it seized again, so I mopped up the fluid and put the torch back on it. Then the rubber hose melted off the fitting, dripping brake fluid that ignited in the torch flame. The fluid dripping out of the fitting burned up into the metal line, and smoke started gushing out.
I looked around for a fire extinguisher and lamented that I'm now that guy who didn't buy one because it wasn't needed yet. Doing the only thing I could think of, I hawked a loogie into a shop towel and stuffed the wet part into the line to smother the fire.
TL;DR: Using the thermal shock trick on a rusty brake line is a bad idea unless you are damn sure the line has nothing in it.