I have never seen the mistake before in a finished lamp, but having mounted hundreds of these filaments myself over the years, I can confirm its something that occurs very easily.
You begin with tungsten wire which passes through a device called the Jazz Machine, which kinks the wire at regular intervals, where the supports will be. It's kinked in one continuous length of a few miles, and so strong is the 'set' applied to the wire, that you can coil it back up on another spool for subsequent use.
The kinked filaments don't coil up on the spool smoothly though and sometimes twist. Then when you unwind them and begin looping the filament over its supports, you always find that the filament has the incredibly annoying habit of suddenly springing around and twisting itself up like this. When it happens you have to take the filament off its supports and begin again. No doubt some production operators got so fed up with this, I can understand that maybe on a Friday afternoon when you've had enough, a few defects will slip through!
James
