Here's a great article on how 3rd party routers and NAT devices cause problems because of problems with their software. In this article Eric Detoc describes how a user wasn't able to connect to a PPTP VPN server, not because of a problem with an ISA firewall, but because of a problem with a NAT device that had a fault PPTP NAT editor. You might think this is a "one off" issue, but it's actually quite common, with several major manufacturers of popular NAT devices leading the way with this problem.
Check out the problem and the solution over at:
Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer