Beware for gross content.
My art teacher told the class a story in second semester last year. She was donating blood at school as a teenager and the Red Cross was there. One of the nurses stuck a needle in her arm with a bag attached to the other end to collect blood. Apparently, they were quite busy on that day, so the nurse had to leave my art teacher to help another person. My teacher kept looking down at her arm and the bag kept filling more and more. When the nurse came back, the bag was gigantic, so both the nurse and my teacher started freaking out. The nurse actually squeezed the bag causing the blood to funnel back through the needle and into my teacher's arm. A gigantic tumor-like black bump resulted on her arm from the built up blood that stayed for over a week.
I didn't like that story.