This is pretty common the bolts that hold the valve cover on are SOFT. The way they are designed there is a shoulder on the bolt. when they are tightened down the shoulder makes contact and thats as tight as the bolt gets, If you keep turning it the bolt just stretches and breaks because it cant tighten anymore. These bolts are only torqued to around 8 Foot pounds. Just barley snug is all they take.
The good newes is is not hard to fix. you need to remove the valve cover, And the cam girdle that the bolt is broken in. Then juse use a drill press and an ez out and it will come right out. I have done the same thing before.
Good tech tip........tighten the bolts will never fix a leak the rubber gasket has shrunk or got compressed and needs to be replaced and the bolts cant put anymore pressure on it because they are bottomed out on the shoulder.
High Velocity Cycles 2005 Ti ZX10R