To improve performance Safari will attempt to prefetch DNS information. In some circumstances this can result in slow or partial webpage loading, or webpage cannot be found errors.
If you are experiencing those problems, apply this tweak to disable DNS prefetching.
Disable DNS prefetching:
defaults write com.apple.safari WebKitDNSPrefetchingEnabled -boolean false
To activate this close Safari, or reboot your Mac.
Return to the default value with the following command:
defaults write com.apple.safari WebKitDNSPrefetchingEnabled -boolean true