I've found nslookup to be practically useless when troubleshooting DNS issues. If I want to actually see what's going on behind the scenes, I normally find a *nix box and run dig instead.

As far as an example problem, when I first got my domain set up it would resolve properly some of the time and not resolve at all the rest of the time. I don't have records of the dig output, but by querying specific servers (starting from 'dig @a.root-servers.net') I could prove it was a configuration error on my ISP's main DNS servers, not an error at the DNS host or on my ISP's resolvers.