"Flashing" Typically refers to a visual defect in a painted surface where different areas of paint have varying levels of sheen. This can cause some areas to appear lighter or darker than others, especially when viewed from certain angles or in certain lighting conditions. In most instances this can be overcome by switching to an ultra flat, anti-reflective ceiling paint, and using a good quality short/medium pile roller to apply evenly. Maintain a 'wet edge' throughout the painting process.