Some of The Common mistakes done while getting the back links are

1) Getting links from irrelevant sites.

2) Links from over crowded links placed sites

3)Posting the threads that already outdated.

4) Getting Non followable links by commenting.

Well, some time when you check backlinks on different search engines, you find a lot links in result but some times when you visit to that particular result of SE you did not find backlink to your site......these type of link are called "no follow" link

Means that your site or link relate to to that but it does not present in the text or in pop up form on that particular link result page....

I say take what you can get.

Sure, you'll want relevant links. High PR links. Blog posts about your site are great too.

But don't snub the less valuable links. All links have some value. A quality link may be worth 100X a lesser link, but that doesn't mean you should pass on the easy to get links.

"Common mistakes done while getting the back links" all points are good and if you keep in mind then you give good result.

Choosing the Wrong Anchor Text:
The anchor text is the text that you use when creating a link. Many people make the mistake of using the words “click here” as the text for the link that goes to their site. Unfortunately, this does not help the search engines to understand what the site is about.