We all know, from recent events, that if your Zan breaks in Shikai it can be repaired (not to 100%). However, if it breaks while in Bankai then you have a big problem.
Q: Didnt Ichigo break Byakuya's Bankai during the Rescue Rukia arc? I distinctly remember both their swords clashing and Ichigo's Bankai cutting through Byakuya's. Then Byakuya saying something along the lines of "you won, my bankai couldnt reach you" or something like that.
Ichigo destroyed only his Bankia Technique, not the Bankai itself. It was like cutting GT, the technique was broken, but nothing happened to the Bankai.