Yeah, they can be very distressing when you normally don't have any problems. I had a blood test recently (2-3 months ago) with some freak results (IIRC I had next to no platelets in my blood), resulting in a panicky phonecall from my GP and me having to take another blood test on the Urgencies ward. In the end all that came up was that my previous blood test had been screwed up, but I'll remember those 2 or 3 hours in a hospital bed waiting for the results as some of the longest of my life.
[/attentionwhoring in someone else's thread]