I have noticed often, that Adobe Muse does not seem to create Line Breaks. Shift-Return most often designates a Line Break in other programs. Muse insists on creating a Paragraph Break, instead of a Line Break; so, whenever I want to force a line break, I often end up with extra space above or below the line, defeating the purpose of a line break.
I know, one shouldn't force a line break on the web too often; but, sometimes it is an efficient way to wrap a short amount of text.