I actually just had this happen to me a few weeks ago. As part of the conditional offer packet they sent me, HR had accidentally printed out the internal job description which showed that I was being offered $20-$30,000 less than had been allocated for my position.
As many have noted, companies will often try to lowball you to begin with, but I was cautioned by my recruiter, who was friends with the hiring manager, that I shouldn't try to argue salary. He said the number was lower for me because of my experience (I had 1-2 years less than was in the job description, but met or exceeded the other requirements). I knew that it wasn't going to work because I was going to walk in the door on day 1 resentful at my employer already.
For a number of reasons, I decided to go with another offer. The non-compete agreement was the main reason I did not want to commit to the job, but they asked what my new salary was and I told them I was going to be receiving $135,000. An hour later the recruiter called me up again and they had "magically" received approval to give me another $15,000, but it was too late already and it wasn't about the money anymore at this point either.
I go with Murphy's law. The HR rep who was sending out that e-mail to me probably just wasn't paying attention and just added every document in the folder without pause. The recruiter actually confirmed that they investigated internally and proper procedure was not followed. But it was more incompetence than malice.
By the way, I was called arrogant, unrealistic, unprofessional and many other things for demanding the salary that I did. The recruiters and companies who responded like that I found to be rude and unprofessional and that was generally the end of our relationship right then and there. I finally found (or rather they found me and reached out to me) a company that respected my skills and paid me more than I had asked for!
It's frustrating, but you know what you're worth and pursue it to the best of your ability. Depending on where you live, it might not be possible to make that kind of money. You may have to move somewhere else to get your targeted salary, but don't give up just because a recruiter told you to.