I'm administrator of modem pool. I'm using mgetty+pppd+pap to authorize
users. Some users share login and password with friends and try to log
twice. To prevent this, I decided to add "session required pam_limits.so"
to /etc/pam.d/ppp and set maxlogins to 1 in /etc/security/limits.conf. It
worked for all other services, but not for pppd. Looking at pppd sources, I
found that it call pam_open_session, but doesn't check what this function
return. With my patch everything is OK

This bug is present even in 5.2 and maybe before. Looks like nobody tried
to use pam_limits with pppd...