This has happened to me about 3 times now.
I've got a lincoln PT185 with a #17 torch, and when i use an alumina type gas lense cup, the front part [where it starts getting smaller] breaks off, its happened on old lenses, and new lenses where i've only gotten a few minutes of welding in.
My cameras dead right now, but if you need pictures to understand, I can get a camera.
What gives?