The polyester thread cutting through cotton thing is not really a myth. When it first became available, polyester thread really was too strong and would cut through cotton fabric over time. Nowadays polyester thread is made with similar tensile properties to cotton. A polyester thread will break when tugged between your hands in the same way a cotton thread will break when tugged between your hands.
Changing times, changing technologies.
The idea that nylon monofilament thread is like "fishing line" is in the same category of non-myths. The YLI nylon monofilament I have is more like baby hair than fishing line, it does *not* melt even when ironed with my Rowenta on high, and I suspect it does not yellow and get brittle over a short period of time (such as 10 years).
Last edited by Prism99; 10-18-2014 at 09:21 AM.