I notice that in
source2e.pdf, the Index section has a running header that intersects with the top of the main text on each page, like so:
(Actually, this seems to happen with some other sections, too, for example "Change History".)
This occurs both in the current version at http://www.tug.org/texmf-dist/doc/latex/base/source2e.pdf (permalink http://www.webcitation.org/5viCxJBr7) and in the ones included with MiKTeX 2.8 and 2.9.
What causes this -- is it something to do with the fact that these PDFs are using the A4 paper size rather than letter?
(And of course it would also be nice if somebody fixed it at some point...)