Pregnancy tests are really very, very, very seldom wrong. I've happily used them with all of mine. A line is a line is a line.
squeaky, when you say that it turned out that you weren't pregnant, did you mean that then your period came? Sadly, these pregnancy tests are so sensitive that they can detect very, very early pregnancies that then don't 'take' (for want of a better word). Hope you are more successful this month.