Worse than the recent joke where a customer shopping for a bathroom scale gets a recommendation for an ice-cream maker:
I was shopping last fall for a book for a neighbor who is suffering from depression, and wanted to give him William Styron's "Darkness Visible" about the author making it through an episode of profound depression.
When I found the book, Amazon's mindless purchase-pattern-matching engine also recommended "Final Exit : The Practicalities of Self-Deliverance and Assisted Suicide for the Dying" by Derek Humphry.
The site no longer recommends those two books together, it was probably not so good for building an ongoing relationship with the customer.....