I've just upgraded a site to 3.5.1 and some pages have errors at the bottom relating to ./admin-footer.php being missing while others are able to find it and the page displays normally.
When I upgrade I remove all the old files and upload new versions so no file is overwritten and therefore possibly corrupt or the old version. I have verified that the file exists and was uploaded as ascii.
Pages that are ok: dashboard, akismet stats
Examples of Pages with errors: