The last few weeks I've been trying to vpn into my office's network. While I could do it at my girlfriend Emily's condo, I could not get it to work at home. I could get it to connect, but it would just hang whenever I tried to do anything. I had almost identical setups between Emily's place and mine (I setup both networks), so I couldn't figure out the difference. Then suddenly vpn stopped working at her place as well and I thought all was lost.
Well I googled around the other day for vpn problems and came across this page http://www.homenethelp.com/vpn/router-client-problems.asp. I tried a few of the things on the site and still no luck, until I saw the last item and realized my home network had a similar schema as my office, 192.168.33.* (thats not really the schema, just an example). So I changed my local network and the sucker worked. I had just changed my girlfriend's router's schema as well (for no good reason), which is what caused vpn to stop working at her place.
Weeks of banging my head on my router and cursing my vpn client (Cisco's VPN v3, which likes to reinstall itself every few days, and I don't know why), and it was a simple router issue the whole time.