Since version 1.3 Magento can use multiple layer of cache, for example a memcached for content cache and session, and a database for the rest. This mechanism work pretty well except when you have a connectivity problem.
If Magento is unable to load a session, the request processing will end immediately preventing Apache from logging it. The only information inside the error log will be an zend_mm_heap corrupted entry.
To ‘fix’ this behavior, you must increase the default timeout for the PHP session handler for a value greater than the TCP re-transmission delay. A 5 seconds value is good enough.