Sometimes a bad DNS entry will be cached and you will need to either flush (or clear) the DNS cache to get rid of it, or wait up to 24 hours for it to be dropped from the cache automatically. You can however FORCE the flush from the system!
In Microsoft Windows, you can use the command ipconfig /flushdns to flush the DNS resolver cache. C:\ipconfig /flushdns
In Mac OSX, you can use the command dscachetil -flushcache to flush the DNS resolver cache.
bash-2.05a$ dscacheutil -flushcache
If you have version 10.5.1 or earlier you need to use the following command: lookupd -flushcache
Unix and Linux flavors
In Linux, the nscd daemon manages the DNS cache. To flush the DNS cache, restart the nscd daemon.
To restart the nscd daemon, use the command `/etc/rc.d/init.d/nscd restart`.
***** IMPORTANT *****
Once you flush the DNS you must open a NEW browser window - do not use an already open one, or you'll just reload the cache.