Shaddows - 11:12 am on May 13, 2013 (gmt 0)
Google always said befor, no backlink can hurt you
No, they said competitors can't hurt you, though they have qualified this slightly.
People have then implicitly or explicitly gone through this logic chain:
1) Competitors can build links to your site
2) Competitors can't hurt you
3) Therefore, links can't hurt you
However, this ignores one salient fact. Competitors typically do NOT spend many months/years slowly adding a small number of forum posts to your site as an act of sabotage. A big rush of links, maybe. But not a slow, sustained drip.
There is nothing to stop links hurting you, and plenty of recent communication from Google to imply that they very much can.
How that balances with competitors' ability to do harm, well that's an open question.