Suppose you have the following configuration

Priv Network A---Windows XP Clients
         |
Linux gateway (iptables NAT etc)
         |
   [Internet]
         |
Linux gateway (iptables GRE and PPTP port 1723 NAT'd and forwarded to internal PPTP server)
         |
Priv Network B---<linux PPTP Server>

Both gateways have ip_conntrack_pptp and ip_nat_pptp modules running and currentish kernels.

In my testing I wanted to find out if the PPTP server on Network B would support multiple connections from Network A. So on a single Windows XP workstation I brought up 2 connections. Everytime I brought the second PPTP connection up on the Windows XP client It would kill the current connection and display a 619 error. In the /var/log/messages log of each linux gateway would be a "kernel: ip_conntrack_pptp: error during exp_gre" error.

However multiple PPTP connections from separate Windows XP hosts on the same "Network A" worked fine.

Testing sometimes brings up unexpected behaviour.