Apparently world leaks occur when some aspect of scripting, etc, on a website cause memory to be allocated by Safari and then fail to be deallocated. It isn't that big a deal. If you want, you can turn on the Safari debug menu and turn off the check for them. For whatever reason, that check has always been off on my computer -- I don't think I've ever changed it myself. AFAIK it isn't considered an exploit or intrusion...just a programming issue.