The Back Link Bomb Mistake

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Once people started to figure out that backlinks produced the sound of a register being opened, they decided to set up multiple websites and interlace all of them with backlinks from each. 

This is not the same as getting friends and neighbors to reference your site. What happens when you own the other sites is that they are inter-related links on multiple sites that you own and operate. While this had the desired effect of boosting your PageRank for a while, it was quickly listed as a black hat trick by search engines and penalized.