I am using CentOS 4.8 I was doing some updates. Now I get "cannot find username for UID 500" here are some outputs,

repeater@irlp's password:
Last login: Sat Apr 2 09:16:27 2011 from 192.168.0.151
-bash: /etc/profile: Permission denied
-bash-3.00$
-bash-3.00$ whoami: cannot find username for UID 500

and should be,
repeater@irlp4745 etc]#





[root@irlp4745 etc]# getent passwd repeater
repeater:x:500:500::/home/irlp:/bin/bash
[root@irlp4745 etc]# getent group 500
repeater:x:500:repeater

Root is Ok.
Seems somehow the name is no longer linked with UID. I searched the web and didn't find any thing specific, I am at a loss.

Solved;
other than user, group chmod was set wrong to /etc and also do a usermod -s to repeater. and set the login repeater to /home/irlp. TNX for the help!

Have you tried useradd or usermod ? I'd add a luser - you can delete him again. That way you get a clear error. Check perms on /etc/profile.
-rw-r--r-- 1 root root 2288 2010-12-04 09:13 /etc/profile
drwxr-xr-x 2 root root 4096 2010-09-09 16:42 /etc/profile.d

I think you may be in danger of developing serious issues. Don't take chances. I'd keep a few terminals open.

Check out the shadow copy files: /etc/shadow and /etc/gshadow
These store the shadow copy used for consistency checks.
Make sure /etc/passwd stays in sync with /etc/shadow & likewise for /etc/group with /etc/gshadow

Please check the permissions of the following files:

/etc/passwd
/etc/shadow
/etc/group
/etc/gshadow