I know in theory a PHP script shouldn't be able to segfault an Apache process, but that's not the case if you let users view hybrid/threaded mode for threads and the threads are very, very large (many thousands of posts). I've just disabled hybrid/threaded mode here to prevent it (I don't think anyone even really uses it). This is the "bug" report I sent to vBulletin if anyone wants more details... http://www.vbulletin.com/forum/project.php?issueid=30158