The fracture surfaces of strip-like single crystal silicon specimens, loaded statically under three-point bending to fracture, revealed an exceptional behavior of surface perturbations generated during the unstably propagating crack. The observed perturbations presented a unique opportunity to study and interpret the relationship between the local crack tip velocity and the mirror and the hackle mechanisms. We show that the high amplitude, more disturbed perturbations are generated at low local crack front velocity, and therefore can termed as a mirror, while the lower amplitude, relatively smooth perturbations are associated with the fastest local crack front velocity, which is actually a hackle mechanism, in contrast to the current knowledge of crack velocity and the associated mirror and hackle mechanisms. (C) 2003 Kluwer Academic Publishers.