+ % The following penalty ensures that the page builder is exercised
+ % _before_ we change the output routine. This is necessary in the
+ % following situation:
+ %
+ % The last section of the index consists only of a single entry.
+ % Before this section, \pagetotal is less than \pagegoal, so no
+ % break occurs before the last section starts. However, the last
+ % section, consisting of \initial and the single \entry, does not
+ % fit on the page and has to be broken off. Without the following
+ % penalty the page builder will not be exercised until \eject
+ % below, and by that time we'll already have changed the output
+ % routine to the \balancecolumns version, so the next-to-last
+ % double-column page will be processed with \balancecolumns, which
+ % is wrong: The two columns will go to the main vertical list, with
+ % the broken-off section in the recent contributions. As soon as
+ % the output routine finishes, TeX starts reconsidering the page
+ % break. The two columns and the broken-off section both fit on the
+ % page, because the two columns now take up only half of the page
+ % goal. When TeX sees \eject from below which follows the final
+ % section, it invokes the new output routine that we've set after
+ % \balancecolumns below; \onepageout will try to fit the two columns
+ % and the final section into the vbox of \pageheight (see
+ % \pagebody), causing an overfull box.
+ %
+ % Note that glue won't work here, because glue does not exercise the
+ % page builder, unlike penalties (see The TeXbook, pp. 280-281).
+ \penalty0
+ %