When a dozen other low authority sites write about the same thing and link to the same site, and they have a pattern of doing this regularly. You tie yourself into the same network by doing the same.
I just wanted to add that you can't hide your link profile, you are who you link to. Likewise those who link to you had better be getting linked to by authority sites or they do you only temporary good, at best. This is the summer in which Google will look at link profiles more than just one hop at a time according to Matt Cutts, webmasters who employ a tiered link approach, especially if any automation is involved, can expect it to stop working very soon according to Matt Cutts's recent videos. I can't wait for it to happen honestly, we'll know it's happening when a surge of questionable authority sites hits the for sale market.