There’s no verse that documents a sin that Mary committed, unlike the Apostles, particularly Paul who goes out of his way to make us all understand the depth of his sins.
All that we have to go on, is what the Angel says to Mary, that “all generations will call you blessed”, something that is not said of anyone else in Scripture. This to me is significant. Scripture doesn’t pull any punches at demonstrating the fallenness of the disciples, and yet it goes out of it’s way to highlight the blessedness of Mary.
Read into that what you will. The argument that Mary proclaims Christ as her Saviour is not the same as when Paul argues that he persecuted the Church, or Peter when he denies Christ, David with Uriah, etc. The list goes on.