Here's one from a Roman Catholic discussion board about Luther being the possible author of Away in a Manger:
For several years, many traditional choir directors have refused to sing Away in a Manger because they think it was written by Luther. A bit of detective work done by researchers at the US Library of Congress finds Luther was not the author. Further, regardless of who the author is, there is no heresy contained within the stanzas but only a sweet song about baby Jesus.
This link provides information to dispel this myth:
So how did a hymn that first appeared in the United States at the end of the 19th century become connected to the 16th-century German reformer Martin Luther?
The culprit who made the false association between “Away in a Manger” and Luther appears to have been James R. Murray (1841-1905), who in his Dainty Songs for Little Lads and Lasses (1887)—a most Victorian-sounding title—called it “‘Luther’s Cradle Hymn,’ composed by Martin Luther for his children and still sung by German mothers to their little ones.” However, no one has uncovered an original German version by the reformer.
Gealy, citing a 1945 article by Richard S. Hill, noted that “illicit inferences” to Luther are partly due to “the association of the carol with the glorification of Luther’s family life as depicted in a series of sentimental engravings done in the early nineteenth century by G.F.L. König . . . [including one that portrayed] Luther with his family on Christmas Eve as frontispiece [for a Christmas book].”
Theophilus Baker Stork (1814-1874), the author of this book, also wrote Luther at Home (1872), in which he stated, “Luther’s carol for Christmas, written for his own child Hans, is still sung.” The irony of this assertion is that we actually have a Luther hymn that may have been written for young Hans, “Von Himmel hoch da komm ich her” (1531), published in Joseph Klug’s Gesangsbuch (1535) and translated by Catherine Winkworth in 1885:
From Heaven above to earth I come,To bear good news to every home;Glad tidings of great joy I bring,Whereof I now will say and sing.
Standards for attribution were much less rigorous before the 20th century. For example, in the 18th century, some works ascribed to J.S. Bach because of his stature were not written by the composer. Nineteenth-century shape-note tunebooks have vexed hymnologists for years as they have tried to discern authorship of specific tunes.
Here is "Luther's Cradle Hymn" from Dainty Songs for Little Lads and Lasses (1887). Note the text does read, "composed by Martin Luther for his children and still sung by German mothers to their little ones."
Luther at Home:
Here is Gealy's comment:
Here are some comments from Roland Bainton's Martin Luther's Christmas Book: