"Let's put carbon fibre in there," says the marketing director. "That stuff's stronger. And I can sell it as a high-end feature."

"Yup, yup," replies the shop foreman. "We can do that. It's a bit pricey though, maybe we could use just a bit of it mixed with the fibreglass."

Fast forward three years, and both men are scratching their heads over why the component- which was, according to the designer, more than strong enough in fibreglass alone- has failed catastrophically even though they added a "better" material.

TL;DR: Mixing different fibres in the same load path can lead to a component being weaker than it would be if only one type of fibre had been used.