MacOSX run a local named caching server. In some reason you may need to refresh it manually to get a resent updated DNS record.
Under Mac OSX 10.8 and above, use the following command using the terminal :
sudo killall -HUP mDNSResponder
Under Mac OSX 10.5 and 10.6, use :
sudo dscacheutil -flushcache
Under Mac OSX 10.4 and below, use :
sudo lookupd -flushcache