FFSAKE I put so much effort into my belt alignment and now this

I put so much effort into getting my belts evenly tensioned or so I thought. I was confident I had eliminated all friction as much as possible and now this WTF. I guess I should replace the belts now presumably. My understanding is that you can get gates ones which are better quality, anyone know where from? Also any tips on how to ensure its doesn't happen again? I scrutinized the pulleys and belt motion so this baffles me.