df = pd.DataFrame({"letters": ["a","b","c","b","c"], "numbers": [1,2,3,1,2]})
correct_mapping={"a":1,"b":2,"c":3}

df[correct_mapping[df["letters"]] != df["numbers"]]
TypeError: 'Series' objects are mutable, thus they cannot be hashed

letters  numbers
3       b        1
4       c        2