Can pregnancy tests be wrong?

Interestingly, it seems like HPTs and even blood tests for pregnancy can be wrong! I am reading this great book (review to come) about a woman who was 6 months pregnant before doctors figured out what was going on (they kept telling her she had a cancerous cyst, it was all in her head--and that she should drink more wine, etc., incredible!).

A quick Google search unearthed 4-5 stories like this (mostly in response to women wondering why they have pregnancy symptoms but negative tests)!

Welllllll.... haha.. I got a negative blood test and SEVERAL negative HPT's... And.. I was definatly preggers... My daughter is 3.5 yrs now.. So.. Blood or urine... No test is ever 100%... (sorry).. haha

Makes you wonder.