DNA
One common error is base mismatch or where the bases are not matched correctly. This is where maybe for example, adenine is not matched with thymine or Guanine is not matched with cytosine. When a cell copies its own DNA a special enzyme called polymerase matches the bases together. But once in a while, there is an error. Usually, the enzyme notices it and fixes it, but just make sure, another set of proteins check what the enzyme has done. If the proteins find a base that was not matched with the right base, they remove it and replace it with a nucleotide with the right base.