How common are false positives?

I'm just a student but I know that I have never had a false positive. In fact, I had a miscarriage and tested three weeks later and was positive. I called the hospital and they said my HPT was so low when I miscarried that there is no chance that the miscarriage was causing the positive and that I had to be pregnant. I am anxious to see what kind of answers you get.

Here is an excellent article discussing the rare occurence of false pregnancy testing and what can cause it. VERY rare but possible. Here is possibly how:

http://www.babyhopes.com/articles/falsepositive.html

hope this helps you.