Photomasks are in widespread use in production of integrated circuits either in the form of reticles or step-and-repeated working masks. The quality of these masks depends on control in both line dimensions and defect density. Unders'tanding of defect generation mechanism has been a topic of extensive study in the past. In this paper, defect caused by over-lapping flashes, exposed by an optical pattern generator on emulsion photoplates, has been reported. It has been observed that pattern generator data tapes produced by computer-aided-design (CAD) systems often contains flashes that overlap in edges or corners. This causes a distortion in line demensions and can be a serious limitation when the design rules in mask layout are quite tight. In this paper, we have discussed the effect of this overlap, its causes and possible remedies. It has been further shown that the removal of overlapping flashes from critical areas can improve the quality of the reticle.