John Battelle comments on gaming a recommendation system as a possible explanation for the embarrassing “similar items” result at walmart.com:

We all know about Google Bombing. But Recommendation System Bombing? That’s a new one to me!

Not new at all. We saw this at Infoseek in roughly 1998. As I remember, a vendor came to us with a system and we showed them how easy it was to spam. I’m sure that Firefly knew about it before then. Any kind of adaptive system (ranking, recommendation) is vulnerable. That is probably why personalization keeps reappearing on WWW engines, the login makes it much harder to spam.

Think like a spammer — if the system reacts to you, you can probably hack it.