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MCAT administrations are very regimented on account of testing and security policies. You should familiarize yourself with test day procedure during the lead up to your exam. The most comprehensive breakdown of Registration, Test Day, and Scoring policies is AAMC's "official policy and procedure guide" MCAT Essentials (2015).
While experiences are meant to be uniform across test centers, things do happen outside of what is planned and expected. In some cases, such as a technical issue, you may be rescheduled for a new testing location or date. Do your best to be patient and flexible. Remain calm and professional, and focus on your exam.
Using MCAT Test Dates with Registration and Score Release dates to help you plan your test day