I’ve always thought it was a terrible idea to stage opera overtures. The music is there to help set the mood for what’s to follow, to allow you to open the magic casements of your imagination and picture for yourself what’s going to happen later—and for the only time to concentrate completely on the music itself. But these days, it’s almost impossible to see any opera performance that doesn’t have a staged overture, and all too often the staging has nothing to do with the music we’re hearing (last season’s Boston Lyric Opera Flying Dutchman was one of the worst offenders in this regard). But it turns out there’s something even worse than staging an overture, and it happened at the Lyric’s new production of Mozart’s Die Zauberflöte (The Magic Flute, at the Shubert Theatre, closed October 13).