What if the typesetting process puts a paper overlength?
It used to be the case that proceedings articles submitted to
a Springer Lecture Notes in Computer Science volume were published
in unchanged form.
Regrettably, this has changed. Since a few years, Springer sends
the LaTeX files submitted by the authors to some copy editing office.
The copy editor inserts a copyright notice on the first page,
checks the bibliographic entries, and possibly makes some further
changes to the formatting of the submission. This has some
undesirable consequences. One of the consequences is that
the authors have to spend an afternoon checking
the PDF proofs that are sent back by Springer
by visually comparing them to the original submission
(and no, the modifications introduced by the copy editors are
not always sensible).
The second consequence is that the copy editing process may
change the number of pages required for the paper.
In fact, I've seen proceedings volumes where virtually every
paper had been carefully
formatted by the authors to fit into exactly 15 pages, 
and where virtually every paper
used 16 pages after the copy editor had inserted the copyright notice.
But in all cases I've seen so far, this was nothing that
the authors or proceedings editors had to worry about.
In fact, making the authors or proceedings editors responsible
for changes introduced by the publisher would be absurd.