What are the limitations to using electrophoresis to verify purity of a protein product?

 SDS-PAGE electrophoresis is used to verify the purity of a protein product. Some of the limitations of this method are:
·     identification of a band on a protein gel is not always reliable because different polypeptides can have very similar molecular masses,
·     band on a protein gel may mask the presence of more than one polypeptide, ·     presence of non-protein residues can affect the mobility of polypeptide and thus cause an error.

What is the example? i’ll be happy to answer if you can tell me : )