Misunderstand the required exams and scores by the university (e.g., GRE/GMAT and TOEFL/IELTS), also, misunderstanding the exams policies, for example, you have a limited number to take GMAT/GRE and TOEFL/IELTS (please visit their websites)
Waiting for the last day for applying – the earlier, the better.
Delaying the Ph.D. application until MS completion. GAs are strongly recommended to start applying for PhD before completing their MS.
Letters of recommendation are one of the most critical components of your application, only use faculty for reference letters, only use faculty who know you well and who you believe will write a strong letter for your academic strength and success is much more important than "character", follow up with faculty.