Although angels are infallible, they are not machines. They have free will and they make their own decisions. Thus they are prone to go against advisory commands. Infallibility, on this level, only guards against direct commands but not advisory ones. Hence the story of the angel Futrus. The Hadeeth is this: "When Imam Hussain (A) was born, Gabriel was ordered to come down to earth with 1000 angels to congratulate the Prophet (S) from Allah (SWT) and from himself. When he came down he passed by an island in the sea which had an angel named FItis (or Futrus) who was one of the carriers of the throne. Futrus was sent by Allah (SWT) to do something and he was slow so his wing was broken and he was thrown to the Island. He worshipped Allah (SWT) there 700 years until the birth of Imam Hussain (A). He told Gabriel "where are you heading?" Gabriel said: "Allah (SWT) has gifted Mohammad (S) with a gift and I am going there to congratulate him for Allah (SWT) and for myself. Futrus told Gabriel: "carry me with you, maybe Mohammad (S) will pray for me", so he carried him. When Gabriel enter to the Prophet (S) and congratulated him, he told him about the situation of Futrus. The Prophet (S) said: "rub yourself on this infant and go back to your place". He rubbed himself against the baby and was able to go back. Then he told the Prophet (S): "your people will kill him but I owe it to him that anytime someone visits him, I will tell him; anytime someone says salam to him, i'll convey that salam and anytime someone prays on him, I'll convey that prayer". And he went back to his place. (Alamali, 138) Also the story was narrated in (Misbah Almutahajid, Basa'ir Aldarajat, Kamil Alziyarat, Rodat Alwa'idheen, Dala'il Alimamah, Almanaqib, Alkhara'ij Wal Jara'ih, Mustatrafat Alsara'ir, and Bihar Alanwar).