Fulltext native search.

Search keywords examples:

(*)

|*|*|

*|keyword

Then you get:

SELECT p.post_id FROM (phpbb_search_wordlist w0, phpbb_search_wordmatch m0) LEFT JOIN phpbb_posts p ON (m0.post_id = p.post_id) WHERE w0.word_text LIKE '%' AND m0.word_id = w0.word_id AND p.post_approved = 1 AND p.forum_id NOT IN (100, 101, 102, 103, 104, 105, 106) GROUP BY p.post_id, p.post_time ORDER BY p.post_time DESC
LIMIT 250;

The important part is:
word_text LIKE '%'

This will cause long execution time on big boards with large phpbb_search_wordmatch table. This will lead to table lock on other tables causing board to crash.

Confirmed with version 3.0.2 and 3.0.7-PL1