mysql error

[محروم.كوم]

i am getting this error on some pages of my vb

Code:
Database error in vBulletin 3.8.2:

Invalid SQL:
Select userid,username,profilevisits From vb_user order by profilevisits DESC LIMIT 5;

MySQL Error : Error writing file '/tmp/MYsCiIDp' (Errcode: 28)
Error Number : 3
Request Date : Friday, June 12th 2009 @ 08:47:47 AM
Error Date : Friday, June 12th 2009 @ 08:47:47 AM
Script : http://www.nannu.info/forum/
Referrer : http://www.nannu.info/the-unforgettable-legends-promo/
IP Address : 117.199.81.42
Username : Gohal
Classname : vB_Database
MySQL Version : 5.0.62
can someone please explain this??? __DEFINE_LIKE_SHARE__