Alright, so it would appear that by merely spamming right click I can get an extremely overpowered form of flight with the hang glider. If I spam right click rapidly in a short period of time, the hang glider will render incorrectly, but I will glide at a rapid pace, the catch here is that it is without altitude loss. Here's a screenshot of the strange rendering:

I'll just keep travelling forward, not losing altitude. It's easy to escape from this, but it's also quite cheaty.

I'm finding the same issue on a FTB:IE:E 2.5.0 wth open blocks 1.5.1. This is a better report than mine. Sometimes when it breaks it affects all players on the server. Sometimes you can't use the glider and once it works you go sonic speeds. Holding down right click increases speeds.