Mysteriously Moving Margins in Word
In Microsoft Word 2008 (and older versions), if you put your cursor in a paragraph and then move a tab or indent marker in the ruler, the change applies to just that paragraph. If your markers are closely spaced, you may have trouble grabbing the right one, and inadvertently work with tabs when you want to work with indents, or vice-versa. The solution is to hover your mouse over the marker until a yellow tooltip confirms which element you're about to drag.
I recently came to appreciate the importance of waiting for those tooltips: a document mysteriously reset its margins several times while I was under deadline pressure, causing a variety of problems. After several hours of puzzlement, I had my "doh!" moment: I had been dragging a margin marker when I thought I was dragging an indent marker.
When it comes to moving markers in the Word ruler, the moral of the story is always to hover, read, and only then drag.