More like a simple minor modification. Due to some concerns about hearing the Glass while in crowded public areas, Google has added a mono earbud to the right side of the device. It will still retain its bone conduction speaker, and the earbud is reportedly removable. To me this seems like a pretty obviously important feature for such a device and its somewhat laughable that this comes a later addition. I don’t really think that it was a design oversight. The earbud looks a little goofy and I don’t think that Glass would have made the desired impression Google had in mind if the bud was initially included.