If you're using
VPNC for VPN'ing with Linux (sudo aptitude install vpnc) and it just won't authenticate, even though you're sure all your information is correct, as a last resort double check that you don't have trailing spaces after any of the lines in your /etc/vpnc/*.conf file. Spaces after any of these lines are significant.
Especially if you're editing your .conf files with vim, this is an easy mistake to make.