Noticed this happening a couple of times while posting tonight (Around 2-3 AM)
First, after submitting the post, I would receive a 504 - Gateway Timeout. After trying again, I would then receive a message like the following:
Could not insert new word
INSERT INTO phpbb_search_wordlist (word_text, word_common) VALUES ('breathed', 0)
Line : 209
File : functions_search.php
However, at that point, the post did make it through in some capacity. Trying again results in a double post, though usually the second post makes it through with no errors.