"Warning: Unknown(): A session is active. You cannot change the session module's ini settings at this time. in Unknown on line 0

see http://bugs.php.net/bug.php?id=26005

it appears to be related to a PHP bug involving out-of-memory situations in a multi-threaded (Apache2) environment; I experience this bug during calls to the cron.php, and the error is very often preceeded by (or near)

Allowed memory size of 8388608 bytes exhausted (tried to allocate 33 bytes)

But I also saw a long string of them during the cron.php call (which includes aggregator runs) while spamassassin's spamd was under a flurry of .sg-domain hits that drove system load over 12.

all of it circumstantial, but I hope it sheds some light on this for others.