Core, no mods happens very often while melee fighting centipedes, but it also happens on humanoid pawns on the waist.'
"Added injury to MechanicalCentipedeBodyFirstRing but it should be impossible to hit it." - this error pops up often.
Additional info looks to be: pawn=(Mech_Centipede# dinfo(def=Cut, amount=20.06719, instigator=Bascullo. angle=315.0).
Steps I took to duplicate:
start new tribal game for 5 pawns
spawn in 5 marine armor/helmets/longswords
spawn in 1 centipede and melee attack it.
Haven't been able to reproduce. Seems like it has already been fixed. Thanks for reporting! ;)