Requires broken data in the question categories. Two categories that refer to one another causing a loop, e.g.

select id, parent from mdl_question_categories where id in (830,882);
id | parent
----+-------
882 | 830
830 | 882
(2 rows)

I know... should never happen, but above is from a client database.

http://git.catalyst.net.nz/gitweb?p=moodle-r2.git;a=commitdiff;h=d4354b8332dbed201bffb28164cc8a696fdd71b8 is a defensive patch that prevents the question_category_next_parent_in() function from recursing into a category it has already seen, thus preventing the infinite stack overflow that causes the segfault.