The first time I gave blood was with school so I went with some friends, and just as I was getting hooked up to the needle one of my friends was done donating and was very nearly unconscious, but as soon as my needle went in and the nurse went to tend to my friend, my needle came out and blood started pouring all over the chair and floor. I let out a meek "Uh, excuse me..." and the nurse turned around to see my arm, the chair, and the floor being painted red rather quickly, so she rushed over and plugged me up as my friend continued slipping in and out of consciousness, but by this time he had another nurse tending to him, so that was OK.
They didn't have enough blood to use; only enough to test.
This was about 3-4 months ago. I should probably go back, maybe it'll go better.