A buddy of mine who is an experienced pilot with over 20 years of experience had this happen with a Golberg Extra 300S. It had been balanced both longitudinally as well as laterally and the throws checked and rechecked.

He was flying along at full throttle and had been doing loops and rolls decided to do a tight loop. The plane pulled up and then it snapped and went real crazy on him. Fortuantely all those years of experience and the fact that he was about 3 or 4 mistake levels high (because it was a new plane) and he was able to save it.

We determined that it was a type of high speed stall, the name of which escapes me right now. Basically the plane is flying straight and level and you pull up so fast the you loose the laminar air flow over the wings and then gravity takes over until you can get it straightened out (if you have enough altitude to straighten it out).

Andy