I mean totally messed them up, last page of the serps in the supplemental index and not appearing in the search results when I search for the domain without the domain extension. All the hallmarks of a 'last page of results' AKA the '-950 Google penalty'.
It seemed like a good idea at the time, after all, you can't hurt a site with backlinks right?
I didn't just blast the sites with low quality backlinks, but I'm sure the way in which I created around 6k links caused such a drastic penalty.
Well, I could have kicked myself when the sites disappeared.
However I soon realised I've learnt a valuable lesson and possibly acquired something far more powerful than a small money site in #1 spot on Google.
I've since removed the backlinks and I'm currently waiting to see if the penalty expires and the sites bounce back. The penalty is an algorithmic penalty, a reply to a reinclusion request confirmed this.
In the wrong hands, this method of backlinking could create carnage for new sites in Google Serps.
Moral of the story? Don't listen to those that say you can't hurt a site with backlinks. Instead, be VERY careful with your backlinking.
Want proof? If you have site less than 6 months old, PM me your domain and main keyword and I'll give you the same backlinks.