When saving an entry, Craft will sit endlessly waiting for the page to load. I've seen this behaviour intermittently in the past - but not for months. It's doing it again now though.
Seems to me to be related to PHP or Apache in some way. I can restart Apache and Craft will then work fine on the first save, but edit and save again and it'll sit spinning forever. The 'hung' save does actually work, you can see changes reflected by checking the front-end in a different browser. However, while the save 'redirect' is hung, you can't browse any of the website, back-end or front, from the browser you were using to edit the entry.
This is only happening on our staging server, not our local machines, though both run the same website and same database.
Staging is Debian Jessie, on PHP 5.6.17-0+deb8u1 and Apache/2.4.10 (Debian). Our local machines are Macs running PHP 5.5.31 and Apache/2.4.16 (Unix).
I haven't found a cause or solution, and there doesn't seem to be anything of use in the craft.log either (though I'm not sure what to look for).
(This post is a follow-on from Craft hangs when updating entry, which never seemed to be answered definitively).