Question

Does Braess' paradox (where adding extra capacity to a network can actually decrease average flow speed) occur in physical systems like fluid flow or electricity?

Answer 1

Here's an example of Braess' paradox occuring in a physical system:
http://mindyourdecisions.com/blog/2011/12/23/video-braess-paradox-demonstrated-with-springs/

Answer 2

I'm not so sure whether that really is an example, but interesting nonetheless.