Segfault during deep recursion
Deep recursion during php clone will cause a segfault:
$ php -r 'class A { public function __clone() { clone $this; } } clone new A();'
Segmentation fault
This can happen during cloning of large or self-referential data structures, such as linked lists. This was filed as a bug in 2009, but marked as "not a bug" because "Infinite recursion crashes. There's no fix for that."
In PHP versions before 5.3, this happens for any deep recursion:
$ php -r 'function a(){ global $i; $i++; print "$i\n"; a(); } a();'
(snip a few thousand lines...)
19162 19163 19164 19165 19166 Segmentation fault