The auto-completion popup which is triggered by C-space (or any other such command), can not be dismissed by pressing backspace. I think this is an unexpected behavior, especially when you are using the "." to trigger the auto-completion command in some language specific plugins.
I am not quite sure if that's a plugin bug or general bug, however I think it's a general one since C-space is also affected. It looks rather odd when the auto-completion popup is still visible at the other end of the file, after deleting a couple of lines...