Module and functions goes missing intermittently
I may have tracked this down to memcache, though it's such a strange error I can't be sure.  Sometimes a group of nodes we put into cache was over the memcache 1mb default limit.

So instead of caching the nodes, I ran them through the theme layer first and reduced the required caching space by about 80%.

This didn't exhibit on my local host as I don't have memcache installed, rather relying on Drupals database caching.  Running some queries like this helped identify the potential problem:

SELECT cid, LENGTH(data) AS Cache_Size FROM cache ORDER BY Cache_Size;