Is it common practice to hire third parties to do code reviews for contractors?

Question

I recently observed some contract offers which included a "code review by third party" clause - the contract would not pay out fully until the code review was completed and it received a pass.

I was surprised, especially considering that these were fairly simple, and small-scale contracts (churning out vanity apps for the iPhone).

Is this kind of third-party code review a common thing to run into when contracting out as a programmer?