I got a vintage bike to commute with (I wanted a heavy one for the exercise) and I was hoping for a flat so I would have an excuse to spend money on a puncture resistant street tire to replace the knobby tire.
After more than a year of hoping for a flat, I got one. While inspecting it I noticed I had two small pieces of glass in the knobs, but the flat turned out to be on the valve neck.
Also, when I took the old tube out, it had what I guess is a Kevlar layer protecting it. I'm not really sure what it is; it is just a brown layer on top of the tube.
It is "glued" to the tube thanks to the amount of time they were together, but it peels off easily. Also, you can see the flat where the valve meets the tube on last pic.
If it is a puncture protection, I must say it was doing a very good job. And I will probably buy regular cheap slick tires and reuse that and be happy with not wasting expensive material every time the rubber wears out. In fact I will continue wondering why tire manufacturers even insert that layer on the tires at all! Unless someone with more knowledge here has an argument against that layer and in favor of modern puncture resistant tires.