Hi,

I have a IBM Thinkpad 600X which is running a 2.6.6 kernel without any 
other patches on a gentoo distro. Nearly every time when I log in at a 
tty as normal user I get this message

Jun 24 16:54:33 paradise login(pam_unix)[8502]: session opened for user 
jens by (uid=0)
Jun 24 16:54:33 paradise kmem_cache_create: duplicate cache sgpool-8
Jun 24 16:54:33 paradise ------------[ cut here ]------------
Jun 24 16:54:33 paradise kernel BUG at mm/slab.c:1372!
Jun 24 16:54:33 paradise invalid operand: 0000 [#1]
Jun 24 16:54:33 paradise PREEMPT
Jun 24 16:54:33 paradise CPU:    0
Jun 24 16:54:33 paradise EIP:    0060:[<c012dc0a>]    Not tainted
Jun 24 16:54:33 paradise EFLAGS: 00010202   (2.6.6)
Jun 24 16:54:33 paradise EIP is at kmem_cache_create+0x3b0/0x40c
Jun 24 16:54:33 paradise eax: dcf56a2f   ebx: dbbbc820   ecx: c0383af4 
edx: d409c000
Jun 24 16:54:33 paradise esi: c02b5ab4   edi: c02a79c0   ebp: c13aed50 
esp: d409df68
Jun 24 16:54:33 paradise ds: 007b   es: 007b   ss: 0068
Jun 24 16:54:33 paradise Process modprobe (pid: 8694, 
threadinfo=d409c000 task=db4a66b0)
Jun 24 16:54:33 paradise Stack: c0000000 00000060 00000000 00000000 
c02e503c c02e503c dce6c0d6 dcf56a2f
Jun 24 16:54:33 paradise 00000080 00000020 00002000 00000000 00000000 
c02e5054 dcf60ce0 dce6c006
Jun 24 16:54:33 paradise c02e5054 c0124cd8 0806df58 0805ac30 4012c4e0 
d409c000 c0103ac7 0806df58
Jun 24 16:54:33 paradise Call Trace:
Jun 24 16:54:33 paradise [<dce6c0d6>] scsi_init_queue+0x26/0x90 [scsi_mod]
Jun 24 16:54:33 paradise [<dce6c006>] init_scsi+0x6/0xb0 [scsi_mod]
Jun 24 16:54:33 paradise [<c0124cd8>] sys_init_module+0x10e/0x1c8
Jun 24 16:54:33 paradise [<c0103ac7>] syscall_call+0x7/0xb
Jun 24 16:54:33 paradise
Jun 24 16:54:33 paradise Code: 0f 0b 5c 05 8c 71 2a c0 8b 6d 00 8b 45 00 
0f 18 00 90 81 fd

If I log in as root, all seems normal. The only remarkable difference 
between a user and root account is, that root uses bash as login shell 
while users use zsh.

What could be the reason for that?

While looking for a solution I did some grepping in /var/log/messages 
and found another call trace:
Jun 24 16:50:53 paradise Linux agpgart interface v0.100 (c) Dave Jones
Jun 24 16:50:53 paradise kobject_register failed for agpgart-intel (-17)
Jun 24 16:50:53 paradise Call Trace:
Jun 24 16:50:53 paradise [<c018ed62>] kobject_register+0x35/0x3d
Jun 24 16:50:53 paradise [<c01d9a12>] bus_add_driver+0x30/0x83
Jun 24 16:50:53 paradise [<c01d9d7f>] driver_register+0x2d/0x31
Jun 24 16:50:53 paradise [<c01945af>] pci_register_driver+0x4e/0x6a
Jun 24 16:50:53 paradise [<dce3a021>] init_module+0x21/0x2c [intel_agp]
Jun 24 16:50:53 paradise [<c0124cd8>] sys_init_module+0x10e/0x1c8
Jun 24 16:50:53 paradise [<c0103ac7>] syscall_call+0x7/0xb
Jun 24 16:50:53 paradise