When i merge thread , i get Database error
 

Database error in vBulletin 4.2.0:

Invalid SQL:
DELETE FROM activitystream
WHERE
typeid =
AND
contentid IN (SELECT postid FROM post WHERE threadid IN (339,251));

MySQL Error : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AND
contentid IN (SELECT postid FROM post WHERE threadid IN (339,251))' at line 4
Error Number : 1064
Request Date : Saturday, June 9th 2012 @ 10:15:04 AM
Error Date : Saturday, June 9th 2012 @ 10:15:05 AM
Script : http://forum.fckya.com/inlinemod.php...eadids=339,251
Referrer : http://forum.fckya.com/login.php?do=login
IP Address : 117.205.1.9
Username : H4ppy!
Classname : vB_Database
MySQL Version : 5.0.95



But if i refresh , or even go back its always done. how do i fix it ?

This is a known bug in the activitystream code: http://tracker.vbulletin.com/browse/VBIV-15189 . If you want you could apply one of the fixes mentioned there.

If you are using the Forum Only version (a similar bug does exist in the Suite if the CMS has been uninstalled and reinstalled a few times), the issue is fixed by 4.2.0 Patch Level 1.