I haven't heard this about the upper thread but the Bernina guy where I got my machine says when you are removing the bobbin from the case you clip thread as close to the casing as possible before removing the bobbin. He said if you continue to pull that long length of thread you will spring the bobbin.
Robin in TX
Sorry about double post. My fingers aren't behaving. Is there a way to remove your own posts? I've tried to "edit" and remove the post but it comes back.