Often single or two layered rectangular pieces have sharp corners on the top or at the bottom.
This is caused in the case of the single layer, 3mm or less, piece by the glass trying to pull up to 6-7mm. In the case of the two layer, ca. 6mm, piece the sharp area is just at the bottom rather than the top.
In both cases the sharp corners on square and rectangular items 3 or 6mm thick can be avoided by nipping a very little off the corners before firing.