I found this article by Larry Lash in Musical America (December 28th - "RSO-Wien Settle the Score with Mahler 6"), but I think that it is only available to those who subscribe. Apparently, most of us have heard Mahler's 6th Symphony in the wrong order. Here's an extended excerpt from the article:
"VIENNA -- The program book for the Dec. 14 performance of Mahler’s Sixth Symphony by Radio-Symphonieorchester Wien (Vienna Radio Symphony Orchestra) at Konzerthaus lists the movements as follows:
I. Allegro energico
III. Andante moderato
An apologetic but urgent insert advised a program change based on new evidence: for almost a century, the movements have been played in the wrong order.
The most-quoted source of the mistake came from Mahler’s widow, Alma: “First Scherzo, then Andante” she instructed Wilhelm Mengelberg when he conducted the work in 1919.
Among personal recollections, Mahler’s own marked-up scores, Alma’s less-than-accurate testimony and printers’ errors, the correct order remained unresolved. The International Gustav Mahler Gesellschaft’s first edition of the Sixth, published in 1963, maintained the Scherzo-Andante order.
John Barbirolli long felt the Andante should come first; when he recorded the Sixth with the New Philharmonia Orchestra in 1968, EMI insisted on issuing the LP with the Scherzo first. Only with the 1996 CD reissue did the label acknowledge that Barbirolli had been correct, accordingly reordering the movements.
Also unresolved was the number of hammer blows in the Finale. Mahler composed three, but later felt their devastating impact too great, too close to tragic events in his life. I once saw his copy of the score, the third blow deleted with the bold stroke of a red marker. Fate would have its way despite the editing pencil: after receiving the blows of his dismissal from Wiener Staatsoper and the death of his daughter, Mahler’s recently-diagnosed heart condition lead to his death at the height of his powers at age 50. For decades, some conductors used three hammer blows, others two.
It’s enough of a mystery to fill a book – which it has, actually, published in 2004 by the Kaplan Foundation, edited by Mahler scholar Gilbert Kaplan and titled “The Correct Movement Order in Mahler’s Sixth Symphony.” A great deal of the material in its 72 pages also was provided by Reinhold Kubik, an IMGM vice-president and editor of the critical edition of Mahler’s complete works.
This performance was the first I’ve heard live in Andante-Scherzo order. The difference is astounding. Rather than proceed from the joyous, wild ride of the closing pages of the Allegro directly into the apocalyptic Scherzo, the insertion of the Andante gives the piece a more logical, theatrical flow. And two of those hammer blows – gut-wrenching no matter how prepared you think you may be for them – are quite enough to make the point."