I bought a domain name from DreamHost back in September 2005. I left them and joined Site5 later but didn't use the domain name. With the 5 domain name quota of Multi-Site account I then moved the domain name to Site5 to make use of the remaining months.
I edited the dns server address in DreamHost, didn't expect it to work immediately. In about 6 hours I can browse the site. 10 more hours later it failed to show up in browser when I viewed it elsewhere.
The Site5 support said it was due to speed of propagation and some dns servers with certain ISP's may not have updated the database. I then learnt from this experience that there is a useful tool here in DNSstuff to check dns and the world network.