UPDATE SET REPLACE() matches but does not change
Look carefully at the query's output

Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0


The expression Rows matched is based on WHERE bug_id=452;. That matched.

When you executed it, the change evidently failed because the short_desc column didn't change because short_desc does not equal 'I don\'t know why my previous comment means.'

To verify that run this:

SELECT REPLACE(short_desc,'I don\'t know why my previous comment means.',
'I don\'t know what my previous comment means.') replace_result
FROM bugs_fulltext WHERE bug_id=452;


If you getback the old value, then the old value was not in the row to begin with.

You should just directly update it (overwrite it):

UPDATE bugs_fulltext
SET short_desc = 'I don\'t know what my previous comment means.'
WHERE bug_id=452;