On a production forum, I found that disabled extensions were not being disabled properly 

https://www.phpbb.com/community/viewtopic.php?f=461&t=2432486&p=14791331#p14791331

This particular server is running APCu cache, but I believe that the root cause of this is not related to APCu (but the symptoms may not be visible with a file cache).

The main internal symptom is that purging the memory cache fails to purge the file cache, because the memory cache object has been constructed without knowledge of the file cache location.

This does not affect all boards, even boards with nearly identical configuration running on the same server.

I do not understand symphony enough to debug how the system gets into this state, and I am not sure whether it is best to add a test for this condition so that the board fails properly, or at least logs an error. (there is usually no externally visible symptom).

I strongly suspect that this issue was also present in 3.1, as I saw some strange behaviour related to memory cache purging on those releases.

I am willing to do more debugging on the board showing this issue, but would need some guidance.