Here lately, I have had to break bad news to people seeking to adjust their status. It usually is because that person falsely asserted U.S. citizenship to their employer.
There is basically one cardinal sin in immigration. Never, EVER, claim to be a U.S. citizen if you are not one. This comes up a lot when an alien states on an I-9 Employment Verification Form that they are a U.S. citizen. They eventually file for adjustment of status and USCIS reviews the documentation and notices that the applicant lied on the I-9. USCIS takes this offense seriously and more than likely the applicant will be deported. There are very few defenses if USCIS seeks to deport someone because they falsely claimed to be a citizen.
Please contact me if you have any questions about an I-9 form or adjustment of status.