False Negative Pregnancy Test

I was wondering if anyone had ever had a pregnancy test be negative when you were actually pregnant? I am a week late, which is abnormal for me because I generally have a pretty normal cycle, but I've taken a few tests and they're negative. My doctor is having me do a blood test, but I won't know the results until next week. We've been trying to conceive for about 6 months now. I'm trying not get my hopes up, but that's easier said than done. I was just curious if this had ever happened to anyone.