Backlinks are the key to high search engine ranking. With good and relevant backlinks from other sites, a site cannot be profitable. Backlinks are part of off-page optmization strategies and in my opinion I believe that off-page is stronger than on-page optimization when talking about SEO.
However, in some cases, your site backlinks can work against you and cause your site to fall from the search engine ranking and in certain cases be deleted completely from search engine index. That means instead of those backlinks helping your site rank well, they would be doing the opposite. These are the ways that can happen:
1. Having too many backlinks from forums and blogs, etc. - If the majority of the backlinks are from forums and blogs, your site could be regarded as a spam site. In most cases, forums are the ways people use to get hundreds of multiple backlinks. Tools have even been developed that could speed up link from these sites.
2. Buying links from other sites - Buying links from other sites will harm your search engine ranking because such links are not placed in the proper places. Have you ever noticed that purchased links are only displayed on a certain page created specifically for that? A good link that counts is one that is within the content of the other site. That is how to identify a link that was not purchased
3. Getting too many no-follow backlinks - Have you been using all kinds of software to get backlinks to your site? Many of those links created by those tools are NoFollow backlinks and cannot help you in any way. However, to make your link building look natural, you can maintain some no-follow backlinks. If you must buy links from other sites, make sure you cover your footprints properly.
4. Too many links from sites with low pagerank - If the majority of your backlinks are from low pagerank sites, your backlinks would be of little help.
5. Excessive Link Exchange - If your backlinks only contain reciprocal links, the backlinks would be regarded as artificial. That means you are exchanging links with the sole purpose of boosting your link popularity.
6. Links not containing site keyword is anchor text - If your backlinks contain anchor text not related to your site, the backlink is of no use. If you are doing blog commeting, make sure you use your keyword as the anchor text.
7. Building thousands of links overnight - Building thousands of backlinks overnight make your site too look unnatural. This is a sign of a spam site. Such baklinks do not count in the search engines and your site can be de-indexed.
8. Using link farms - Search engines hate link farms because it is believed that such farms don't provide any benefit for internet users. If you don't know, link farms are a combination of sites that link each other with the main aim of increasing their link popularity. In most cases, sites are not related and they contain little or no content that could benefit internet users.
9. Link from duplicate content, p-orn, banned and illegal sites - Links from these sites make your site to be regarded as one of them and that you have no value to offer internet users.
10. Links from unrelated sites - If most of your backlinks are from sites not related to your website, those links will not help your ranking. Even if you are using tools to discover auto-approve sites, use keywords to filter the sites to those within your site niche.