|Project:||Memcache API and Integration|
I was running pressflow 6.19, the latest 6.x-dev version of this module, and the php memcache (not 'd') library v3.0.1. Today I deployed some new code, including some new hook_theme() entries, and the theme registry wouldn't load them. I cleared the cache several times, confirmed that the module was running, etc. The exact same database and code on a local setup worked fine, so I concluded it was probably memcache related.
Reading through all the confusing documentation about the memcache and memcached plugins for php, and seeing that the *d variant was recommended with this module, I decided to switch. (That was a pain in itself, I'll write a separate blog post about that.) I finally got that working, cleared the drupal cache, and the theme registry was fixed.
I don't know what the root cause was, but switching extensions seems to have done the trick. Maybe it's related to #616282: Fatal error opening required theme.inc in content.module. I'd be curious if anyone else has a similar issue with the memcache (non-d) extension. Presumably it wouldn't be limited to the theme registry, since that's just one arbitrary cache item out of many.