Here's one I recently came across on the CARM boards: When did Mary become queen of heaven? After a Lutheran participant admitted Luther could be wrong about believing particular Marian doctrines, a defender of Rome asked, "... have any of the various Lutheran denominations denounced their founder's acknowledgment that Mary is Queen of Heaven?" The assumption put forth is that Luther, his entire life, taught, believed, and was overly explicit that Mary was to be regarded as "Queen of Heaven." As we'll see, this assumption is not supported by the extant documents. The "various Lutheran denominations" have nothing to denounce about Luther using the title "Queen of Heaven" in regard to Mary.
The Research Methods of Rome's Defenders
If one does a simple Google search on this, it becomes readily apparent that many of Rome's defenders haven't done any meaningful (or in some cases, actual) research into Luther's use of the title "Queen of Heaven." Consider this example:
Modern Protestants object to calling Mary as “Queen of Heaven” because according to them, the title is pagan and is hateful to God. Before I respond to this to the issue, I just wish to remark that Martin Luther, the originator of Protestantism, Martin Luther admitted that the title “Queen of Heaven” is “a true enough name and yet does not make her a goddess.” In fact, the instigator of the Reformation went as far as calling Mary “more than an empress or a queen.”
 Jaroslav Pelikan, ed., Luther’s Works (St. Louis: Concordia) 24:327, cited in Fr. Mateo,Refuting the Attack on Mary (San Diego, CA: Catholic Answers, 1999) p. 67.
 Jaroslav Pelikan, ed., Luther’s Works (St. Louis: Concordia) 36:208; 45:107, cited in Fr. Mateo, Refuting the Attack on Mary (San Diego, CA: Catholic Answers, 1999) p. 110.
While I don't plan on purchasing Fr. Mateo's book to check out these secondary citations, both footnotes presented here are erroneous. Both footnotes claim to be to the English edition of Luther's Works. Footnote #1 is actually from LW 21:327 (Luther's exposition of the Magnificat), not LW 24. It's also interesting that the quote from Luther doesn't end at the word "goddess" (as will be explained below). In Footnote #2, LW 36:208 has nothing to do with Mary. LW 45:107 appears to be a botched citation from the Weimar edition of Luther's works. The "empress or a queen" comment is often documented as WA 45, 105, 7 to 106, 1. The quote probably made its way into cyberspace by one of Rome's defenders using Max Thurian, Mary Mother of the Lord, Figure of the Church (London: The Faith Press, 1963), p.80. We'll see with this quote that Luther isn't calling Mary "Queen of Heaven" here.
The Salve Regina and the Regina Coeli
Of the works of Luther that I've dealt with over the years, I rarely have come across Luther using the title "Queen of Heaven" in regard to Mary. The reason why is because "Queen of Heaven" was directly associated with the Salve Regina and the Regina Coeli.* The Salve Regina states:
Hail, holy Queen, Mother of mercy, our life, our sweetness, and our hope. To you we cry, the children of Eve; to you we send up our sighs, mourning and weeping in this land of exile. Turn, then, most gracious advocate, your eyes of mercy toward us; lead us home at the beginning and show us the blessed fruit of your womb, Jesus: O clement, O loving, O sweet Virgin Mary. Amen.Also relevant to Luther's time period was the Marian hymn, the Regina Coeli:
Queen of Heaven, rejoice, alleluia. The Son whom you merited to bear, alleluia. Has risen, as He said, alleluia. Pray for us to God, alleluia. V. Rejoice and be glad, O Virgin Mary, alleluia. R. For the Lord has truly risen, alleluia. Let us pray. O God, who through the resurrection of your Son Jesus Christ gave rejoicing to the world, grant, we pray, that through his Mother, the Virgin Mary, we may obtain the joy of everlasting life. Through Christ our Lord. Amen.It was these very sort of examples of medieval Mariolatry that Luther was explicitly against and vocal about throughout his writings. As early as 1522, Luther stated:
Here I must say a few words about the song which is called Salve Regina. It is a great blaspheme of God. For it says, "Hail you queen of mercy, our life, our sweetness and our hope." Is that not too much? Who could justify calling her our life, sweetness and mercy when she is satisfied to call herself, a "handmaiden of the LORD?" Now people sing that prayer in every corner of the world and also the bells ring out, and still today in nearly every church the Salve Regina is, unfortunately, retained and sung.In the sermon of March 11, 1523 (yet untranslated in English, documented by William Cole) Luther wants "devotion to Mary" "entirely abolished because of the abuses" (WA 3, 312 ff; cf. Cole, 189). Cole says, "It is quite clear that [Luther] includes the Salve Regina as one of the abuses" (Cole, 189). Cole also documents another sermon (1524, WA 15, 115, 13f) in which "Luther refers to the Salve Regina as blasphemous inasmuch as the name of life and hope is taken away from Christ in favor of Mary and Mary is made into a goddess with her feasts and antiphons" (Cole, 189). Also documented by Cole is that Luther attributes the title "queen of mercy" to the church rather than something which is to be sung about Mary (Cole, 189; LW 12:261). Cole points out though that Luther is not necessarily against calling Mary a "Queen," but that the Roman church had made her into an idol (Cole, 189-190).
It is the same with the Regina Coeli, which is not much better, in which she is called the queen of heaven. Is that not doing Christ a disservice when you account to a creature what only belongs to and is proper to God? So forget these ungodly and unchristian words. I will gladly concede that Mary prays for me, but I deny that she must be my comfort and my life. Your prayer on my behalf is also just as precious to me and hers. Why? If you believe that Christ dwells just as much in you as He does her, your prayer can help me just as well as hers. [The Festival Sermons of Martin Luther, 105-106].
Mary is "more than an empress or a queen"
As mentioned above, this quote was probably taken from a secondary source. The actual source, WA 45:105, 7 to 106, 1 can be found here. I know of no official English translation for this Latin / German page. Of the secondary translation that is available, Thurian states:
"...then on another Feast of the Visitation, July 2.1537, Luther said: 'When the Virgin received the acclamation of Elizabeth as being the blessed Mother of God, because she had believed and because all was coming to pass as the angel had spoken, she was not filled with pride by this praise which no other woman had ever yet spoken to her—this immense praise: "No woman is like unto thee! you are more than an empress or a queen! you are more than Eve or Sarah; blessed above all nobility, wisdom or saintliness!" No, she was not filled with pride by this lofty, excellent and super-abundant praise ...' " [Weimar, 45: 105, 7 to 106, 1].What's interesting to me is that Luther is not calling Mary the "Queen of Heaven" here, but is rather embellishing the praise given to Mary by Elizabeth, and even Elizabeth (in Luther's words) isn't calling her "Queen of Heaven" but saying Mary is "more than an empress or a queen."
"Queen of Heaven" is a true-enough name...
The only explicit positive quote (I know of) in which Luther refers to Mary as "Queen of Heaven" comes from Luther's treatment of the Magnificat (1521). There Luther states:
It is no valid argument against this to cite the words of the hymn “Regina coeli laetare,” “Whom thou didst merit to bear,” and again, “Whom thou wast worthy to bear.” For the same things are sung about the holy cross, which was a thing of wood and incapable of merit. The words are to be understood in this sense: In order to become the Mother of God, she had to be a woman, a virgin, of the tribe of Judah, and had to believe the angelic message in order to become worthy, as the Scriptures foretold.28 As the wood had no other merit or worthiness than that it was suited to be made into a cross and was appointed by God for that purpose, so her sole worthiness to become the Mother of God lay in her being fit and appointed for it; so that it might be pure grace and not a reward, that we might not take away from God’s grace, worship, and honor by ascribing too great things to her. For it is better to take away too much from her than from the grace of God. Indeed, we cannot take away too much from her, since she was created out of nothing, like all other creatures. But we can easily take away too much from God’s grace, which is a perilous thing to do and not well pleasing to her. It is necessary also to keep within bounds and not make too much of calling her “Queen of Heaven,” which is a true-enough name and yet does not make her a goddess who could grant gifts or render aid, as some suppose when they pray and flee to her rather than to God. She gives nothing, God gives all, as we see in the words that follow. Luther, M. (1999). Luther’s works, vol. 21: The Sermon on the Mount and the Magnificat. (J. J. Pelikan, H. C. Oswald, and H. T. Lehmann, Eds.) (Vol. 21, pp. 327–328). Saint Louis: Concordia Publishing House.Here Luther allows "Queen of Heaven" to be a "true enough name" but qualifies it that even if this name is applied, Mary is not "a goddess who could grant gifts or render aid, as some suppose when they pray and flee to her rather than to God. She gives nothing."
I anticipate this response from a defender of Rome: Yes, Mary is not a goddess. We agree with Luther. Notice though, the Mary of Luther and the Mary of 16th Century Romanism are different, for in that view, Mary is someone to pray to and flee to who grants gifts- hence, what Luther would call, a goddess. According to Luther, by pouring more into the term "Queen of Heaven" (like the defenders of Rome do), "we can easily take away too much from God’s grace, which is a perilous thing to do and not well pleasing to her." In other words, when Luther here says "Queen of Heaven" "is a true enough name," he does not mean the same thing Rome's defenders do. If there's any agreement here between the defenders of Rome and Luther, it's only surface level.
Keep in mind as well, Luther's exposition of the Magnificat was seen in his day as an attack against popular Marian piety, and is a transitional work in Luther's Mariology not entirely reflective of his later thought (Roman Catholic scholar Thomas O'Meara, reaches the same conclusion: Mary in Protestant and Catholic Thought, 116-117). In chronological order, Luther's 1521 admitting a use of "Queen of Heaven" is followed by 1522's "doing Christ a disservice" if one uses the title. Then for the rest of Luther's career, the Salve Regina and the Regina Coeli were to be avoided as blasphemous.
*I'm indebted here to William Cole's article, Was Luther a Devotee of Mary? Marian Studies XXI (1970), 188-190.