Common examples include:
Many old books are scanned but not proofread. Optical Character Recognition (OCR) mistakes ප (pa) for ව (wa) or ත (ta) for ප (pa). This changes the mantra's meaning entirely. Always choose manually typed Unicode PDFs.
Do not combine a Hindu Om Namah Shivaya with a Buddhist Namo Tassa in the same session without proper guidance. A good focuses purely on Theravada Buddhist Pali verses.