Wikipedia users did get nice errors like:
Fatal error: Call to undefined function: peplaceinternallinks() in /usr/local/apache/common-local/php-1.5/includes/Parser.php on line 812
Table 'nlwiki.recemtchanges' doesn't exist (10.0.0.101) UPDATE `recemtchanges` SET ...
In both cases code was absolutely normal and after rogue Apache servers (different ones!) were restarted, those glitches did disappear. No ECC errors reported… what should be blamed? OS? Hardware? Apache? PHP? APC? Ummm….