I got the following error when upgrading a site I hadn't used for several months.
I had a quick search for this issue but couldn't find any other reports. My appologies if there is already a bug for it.
.... or if it is just my installation.

Here's what I got during upgrade:

Language import utility: en_us

cURL request for "http://download.moodle.org/download.php/direct/langpack/2.4/languages.md5" failed, HTTP response code: HTTP/1.0 404 Not Found

line 1285 of /lib/filelib.php: call to debugging()
line 486 of /lib/componentlib.class.php: call to download_file_content()
line 437 of /lib/componentlib.class.php: call to component_installer->get_all_components_md5()
line 369 of /lib/componentlib.class.php: call to component_installer->get_component_md5()
line 277 of /lib/componentlib.class.php: call to component_installer->need_upgrade()
line 779 of /lib/componentlib.class.php: call to component_installer->install()
line 640 of /lib/componentlib.class.php: call to lang_installer->install_language_pack()
line 45 of /admin/tool/langimport/lib.php: call to lang_installer->run()
line 1403 of /lib/upgradelib.php: call to tool_langimport_preupgrade_update()
line 1466 of /lib/upgradelib.php: call to upgrade_language_pack()
line 275 of /admin/index.php: call to upgrade_core()