What black hat SEO mistake cost you the most?

Golden Hawk said:

Hi everyone,
Everyone talks about wins but failures usually teach more. What is one black hat seo mistake that ended up costing you rankings, time or money? Looking for real experience so others can avoid making the same mistake.

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One common mistake that people tend to make is using a successful technique as an absolute system. The testing process will yield positive results due to its small footprint, but if you try to scale and not underestand what exactly leads to your detection, everything is doomed.