This paper introduces some new types of optical fault attacks called fault masking attacks. These attacks are aimed at disrupting of the normal memory operation through preventing changes of the memory contents. The technique was demonstrated on an EEPROM and Flash memory inside PIC microcontrollers. Then it was improved with a backside approach and tested on a PIC and MSP430microcontrollers. These attacks can be used for the partial reverse engineering of semiconductor chips by spotting the areas of activity in reprogrammable non-volatile memory. This can assist in data analysis and other types of fault injection attacks later, thereby saving the time otherwise required for exhaustive search. Practical limits for optical fault masking attacks in terms of sample preparation, operating conditions and chip technology are discussed, together with possible countermeasures.