Makers beware!
Much like with common household chemicals used for cleaning, such as bleach and ammonia, improper use of these can produce e.g. chlorine gas, which while harmful is generally not lethal. Things get much more serious with brake cleaner, containing tetrachloroethylene. As explained in the video, getting brake cleaner on a rusty part to clean it and then exposing it to the intensive energies of the welding process suffices to create phosgene.
The point of brake cleaner is that it’s not supposed to leave a residue. It’s not actually meant to be a general parts degreaser, but rather a braking surface cleaner. But being able to remove oils without additional water has made it the WD40 of mechanical cleaners. Rarely the right product , but often right enough.