Quoting Chroma Hex [+1]:" Those are a lot of different factors....and it's been over two years since it last happen. I've never heard of a chemical pregnancy though...I'll have to go look that up. "
And yes there are a lot of different factors. Women will get a negative, leave the test, come back in a few hours, see that an evap line has formed and believe it to now be a positive test, when in fact, it's just an evap line.
Also, some women pee into a cup first and dip the test. Well if you pee into a cup that has recently been washed and might have soap residue in it, the soap residue can produce a "false" positive.
Like I said, it's usually "user" error. Either the woman did something wrong when taking the test or when reading.
OR a chemical pregnancy, which again is not a "false" positive.