Another cause is pre-formatted text. Lines beginning with whitespace are considered pre-formatted by the wiki, similar to the <pre> tag in HTML. This can happen easily when pasting text from another source, so check those cut'n'paste results carefully.

Why do pages on this wiki not format correctly?

Some pages do not format to fit the width of the window. This is because they contain unsplittable lines which set the minimum width of the text much greater than the width of the window.

These lines appear to be caused by @@typewriter@@ formatting directives. Apparently the wiki code will not split such a line. To avoid this don't set @@ formatting for more than a few words. Simply stop and restart with a pair of @@ directives to give the wiki a chance to line-break.