• a foolish person who openly acts foolish, even if he reports the most narrations;
  
• an adherent of bid’ah who calls to his desires;
  
• a person who lies, even if I don’t accuse him of lying in hadith;
  
• and a righteous pious worshipper who does not accurately retain what he narrates."

Adh-Dhahabi, Siyar A’laam Al-Nubalaa' in his biography of Imaam Maalik.