White spots appear in tissue sections after deparaffinization, and staining is irregular. Which is a likely cause?

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Residual water in the tissue is the likely cause because staining depends on proper removal of paraffin and full hydration of the sections before dye uptake. After deparaffinization, tissues are rehydrated so that aqueous and alcohol-based stains can penetrate evenly. If water is trapped inside the tissue, stain solutions cannot uniformly penetrate those areas, so some regions take up little or no dye and remain pale or white, while surrounding areas stain normally. This creates white spots and an overall irregular staining pattern. In contrast, surface drying issues or incomplete dewaxing would produce different, more surface-level or diffuse artifacts, and a filter problem would cause precipitates rather than localized tissue-wide unstained patches. So the presence of water left in the tissue best explains the white spots and irregular staining.