When Do Poynting Vector and Energy Propagation Differ in Electromagnetic Waves?

[arash1r]

I have a question: when the poynting vector and energy propagation in electromagnetic waves are in the same direction and when are in different direction?

 

[mfb]

The Poynting vector is the direction of energy flow, I don't see how it could point in the opposite direction. Maybe metamaterials have some strange properties here.