Performing Fardh Salaah behind someone performing Sunnah or Nafl Salaah
Q: If one wants to pray a Fardh Salah in Jamaat, can one join a person who is praying a Sunnah Salah? What if one joins a person who is praying a Sunnah salah, but you mistakenly assumed that he is praying a Fardh?