It's always annoying when you watch a film and you're amazed to hear an actor's singing voice, impressed by how well they can hold a tune, only to find out that it isn't them at all, and they've been dubbed. Billie Burke, in the role of Glinda, had the opposite problem.
Burke sang every line of every number she recorded, but viewers of the film were for some reason convinced that a singing double had done it. There was no Google back then, so no way to easily clear up the mystery. Burke had a high pitched, very well-spoken voice - very dissimilar to the voice she sang in - which may have explained the confusion. Burke had actually worked with Judy Garland before, playing her mother in the ironically titled 'Everybody Sing', which had been released only a year earlier.