Spontaneously-formed flakelets like eraillures are sometimes called ‘spin-off’ flakes.  The primary crack in flake-making actually progresses through the stone in minute ‘tongues’ (hackles or lances) emanating from the fracture front.  These tongues join up as the fracture front progresses.  Towards the centre of the fracture front, where the tensile stress is closest to a right angles to the fracture surface, the space between the tongues is microscopic and the fracture plane appears smooth and continuous to the naked eye.  At the edges of the crack, however, the tensile stress is more oblique, and the tongues do not join up as cleanly because of greater displacement between them, and this is how hackles are created.  One place where this oblique stress occurs is around the periphery of the bulb of force.  The bulb presents a zone of high mass ideal for flake propagation, and an eraillure is formed when a hackle propagates across it. 

Usually only one eraillure is formed when a flake is detached, but sometimes two or more may overlap.  The eraillure flake either pops off along with the flake or remains stuck to the negative flake scar on the core.  Eraillure flakes were selected in Mesoamerica for use as beads,  or were used as eye insets in sculptures, but mostly eraillures are part of the discarded debris from toolmaking.  Eraillures are only formed in conchoidal initiations and are common on conchoidal flakes struck by hard-hammer percussion or detached by pressure, although not all conchoidally-initiated flakes have eraillure scars.  Eraillures never occur on bend-initiated flakes.