There are two types of people reading this right now, (1) those who are like me and cannot stand when this happens and (2) those who really don't know it is an issue.
What am I talking about?
You go to a specific URL on your mobile device, an article or a landing page. Then all of a sudden, you are redirected to a home page of a mobile version of the web site - which has very little to do with the content you want to reach.
This is not only horrible for your end users but it is very bad for searchers.
Google's John Mueller wrote in an ongoing Google Webmaster Help thread:
However, one issue that I see is that you're redirecting all mobile users to http://www.ifly.com/mobile , regardless of the URL they tried to access before. This results in us not being able to pick up your content properly with our smartphone crawler. If you wish to promote your app, I'd recommend using the method mentioned over here.
Don't do this to your users and if you do, keep in mind, it isn't good for bots either.
Forum discussion at Google Webmaster Help.