I have a 2-column Word 2003 document, and on the last paragraph of the right
column of one page, the word wrap/line break behavior changes in the middle
of a paragraph.  I go from a 40(ish)-character line to a 30(ish)-character
line, with no codes, paragraph formatting changes or margin changes in
between.  It's hard to describe, but imagine the lines breaking to avoid a
(nonexistent) blank picture in the lower right corner of a page.  

If I cut the text and paste it back in the same place as unformatted text, I
get the same result.  

If I paste it in elsewhere I get consistent 40(ish)-character lines.

If I reduce columns to 1, the last 5 lines or so of the first page of the
document still break inconsistently.