What does it mean when you get a "shadow line" on the first pregnancy test but negative on the second one taken?

Likely not pregnant: If the first test was equivocal and the second one was negative, then you're likely not pregnant. You can always resort to a third one or a blood test. There's a remote possibility of the described scenario in the case of a miscarriage where decreasing beta-hcg hormone gives weakly positive test but as the level thereof fell to normal, the subsequent one is negative.