More often than not, the doctor's aide who checks my blood pressure during doctor's visits screws it up.
I had that happen once. I was wearing a sweater, and as I started rolling up my sleeve she told me to not bother. My doctor came in and noted that my blood pressure was high. I told him the problem was a training issue.
It seems that most doctor visits I go to, they NEVER do a BP reading correctly. They always take the reading over my sleeve, ALWAYS ask me 50 questions as soon as the test starts, don't wait a few minutes after I've walked across the giant facility, always have the cuff too tight. Those automatic BP machines are the absolute worst for pumping the cuff until it actually hurts.
Before & after a tooth extraction, the assistant started to take my BP. I'm always cold in dentist offices, so I was wearing a thick coat, rated for 20 below zero. She wrapped the cuff around my arm, over the coat. I said, "Shouldn't I take the coat off?" She said, "No, it's not necessary." I just chuckled.