First, the Patent Office will perform an "initial examination." During this phase, they will look for everything from trivial grammatical errors up to more serious issues such as confused writing and hopelessly drafted claims. All of these issues will likely occur in Inventor-written applications. Straightening these issues out, if possible, can cost thousands of dollars.
Eventually the Patent Office will perform a substantive examination. At that time, the claims and written description come under severe scrutiny. If the claims do not measure up (even seasoned Patent Attorneys have issues with claim drafting) they will be rejected as being unclear or missing the point(s) disclosed in the application.
Of course, prior art will raise its ugly head at this point. And the best defense against prior art is having a detailed, thorough application. Otherwise, no recourse might exist to argue around close prior art.