Based on OP comment about showing the hex to the clas, my bet is the perp opened the file in notepad, jammed on the keyboard (ascii characters) or worse yet, ty

Based on OP comment about showing the hex to the clas, my bet is the perp opened the file in notepad, jammed on the keyboard (ascii characters) or worse yet, typed a message proceeding or following the content (depending on the format), making it very obvious that it was an intentionally introduced error.