On my site a user just reported that they has this error when trying to post a comment:
Question2Answer query failed:
INSERT INTO qa_contentwords (postid, wordid, count, type, questionid) VALUES (46754,1181,1,_utf8 'C',46728),(46754,1488,1,_utf8 'C',46728),(46754,14,1,_utf8 'C',46728),(46754,1022,1,_utf8 'C',46728),(46754,809,1,_utf8 'C',46728),(46754,491,1,_utf8 'C',46728),(46754,66,1,_utf8 'C',46728),(46754,49,1,_utf8 'C',46728),(46754,104,1,_utf8 'C',46728),(46754,82,1,_utf8 'C',46728),(46754,5403,1,_utf8 'C',46728),(46754,13574,1,_utf8 'C',46728),(46754,386,1,_utf8 'C',46728),(46754,11,1,_utf8 'C',46728),(46754,268,1,_utf8 'C',46728),(46754,276,1,_utf8 'C',46728),(46754,3075,1,_utf8 'C',46728),(46754,47,1,_utf8 'C',46728),(46754,1058,1,_utf8 'C',46728)
Error 1213: Deadlock found when trying to get lock; try restarting transaction
After they commented some errors/notices started appearing on the site in their user profile and All Activity:
After I looked in the database to find the question it seemed that the comment's parentid got set to another comment, which was hidden. I don't know if comments should ever have comments as parents, or if this was something to do with it being hidden.
On the question page itself the hidden "comment" was actually showing as a hidden answer even though it was "C_HIDDEN" in the database. Here are all the posts on that question, maybe it will make sense to someone else:
