When I login into my normal user account under runlevel 3, the following error message is printed to the screen:

Warning: add_groups: Invalid Argument

I'm not sure where this is being generated. I don't see the same error message when I login using root.

I think you'll find it's because you belong to more than 18 groups.

A quick search turned this up

http://www.linuxquestions.org/questi...ing-in-749348/

Aha - I've been getting this message all the time when logging in as root - but my normal user is OK. I've just ignored it but I'm glad someone else has noticed this too. Interestingly I get this :
ed is my normal user so maybe the 18 groups that root is a member of is causing problems. Thanks mRgOBLIN for pointing this out. Maybe time to knock my root user out of a group or two. Is this 18 limit easily changeable?

Yes further down that thread there are some suggestions.. your mileage might be an unknown variable though =)

Thanks. It looks like my searching needs improvement. :-)

Thanks again for the insight - I've looked through the thread and the links seem to be relate to NFS - though admittedly I just briefly scanned them - nothing seems relevant to a non NFS scenario. In any case, as a test, I've created another group and added my root member to it. Now I get this when logged in as root:
so, although I am getting the annoying warning my root user is still a member of 19 groups, as expected. All in all I think I'm going to be auditing my group memberships as certain programs, (kismet, kvm, etc), have created there own on installation - not neccesarily a good thing, (or maybe it was me on their installation - can't remember). To me a limit of 16 seems quite realistic in any case.
Me too.

Edit: Apologies, after giving the original thread closer inspection it seems rworkman may have pointed out a way to rebuild shadow to prevent the warning. Again, I think I'll check through the groups I have at the moment and limit them that way. Thanks again mRgOBLIN, rworkman and rmjohnso.