Speaking of hacks, I've found a few websites whose DNS cache was poisoned, allowing the hacker to divert a percentage of their search traffic! It's not as common as someone inserting parasite content/links or an iframe hack - but it really creates a mystery when it does happen. "How can my stats disagree this much?"
The fix? Check your DNS settings and fix the errors you find. There are many good tests available onlne.
Reference thread: DNS Cache Poisoning [webmasterworld.com]