Very rare..... I suggest you search the forums, I have never seen applicants get denied after they get pre arrival... Why would the officer send pre arrival if you are not approved? This cost a lot of money for the government to finance... It's not a free service. Don't forget that the ones who got pre arrival, they also have background check in process at the same time of eligibility... So, they already know that your background is clean. Also, the police certificate also shows if the applicant has a criminal past or not.