Hello,
I think there is a bug in the "Object Cache" when you use the "W3 Total Cache" plugin on 2 (NOT-multisite) different WordPress installs installed on the same domain.
Indeed, I have a main blog at the root of one of my domain (/) and another blog set up in one folder (/folder/). They use a different set of files and a different database.
I enabled Database/Page/Object caches (through memcache) on both blogs. Sadly, after that, the one in the folder was malfunctioning. Indeed, for an unknown reason, it was picking up the options (the ones stored in wp_options table) from the blog in the root folder.
I quickly noticed that the problem was from the object cache (once disabled, the problem disappeared): it seems the object cache is unable to detect that there is more than one blog sharing the same cache on this domain and so assumed the values in the cache where from this blog and used them. Because the blog in the root folder has a lot of traffic, it was probably the first one to populate the cache...
The database cache is not impacted at all by the problem.
Is this a known bug? Is there a workaround to use the object cache on both blogs?
Thanks,
Tristan