I had seen major Tbelt failures which destroyed the engine caused by BOTH the harmonic balancer coming off or the main crank pully bolt backing out.
A: the harmonid dampner crank pulley gets loose....wobbles a bit, cuts into the plastic timing case like butter and either slices of AC belt or chunks of plastic case get between the Tbelt and pulleys and cause the jump & carnage.
B: the main crank bolt backs out (the one that has the 1/2" square hole) and allows the crankshaft timing belt pulley to walk back off the crank and when it gets about halfway off, the key aligning the pully/crank will snap and allow the two to rotate..causing carnage.
Either way is bad mmmkkkk /ubbthreads/images/graemlins/rofl.gif