I have a question for you. Does the Roman Catholic Church teach that Mary died or did she ascend into heaven as did Christ? Since death is the payment for sin, and she was sinless, it would seem necessary that she not suffer death.
Pope Pius XII 11/01/1950
"44. For which reason, after we have poured forth prayers of supplication again and again to God, and have invoked the light of the Spirit of Truth, for the glory of Almighty God who has lavished his special affection upon the Virgin Mary, for the honor of her Son, the immortal King of the Ages and the Victor over sin and death, for the increase of the glory of that same august Mother, and for the joy and exultation of the entire Church; by the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory. "
You found another one heh - this isn't done often you just happened to ask about two instances where papal infallibility has been invoked and I provided you a 3rd as a means of example.
This site will give you information on Our Lady's Assumption
http://www.catholicherald.com/saunders/04ws/ws040812.htm