False positives can come from reading the test after the specified time (depending on the brand the reagent from the control strip can bleed back into the test area causing a second line). False negatives can be caused by urine that is too dilute. True positive can be followed by a true negative in the case of miscarriage, which should show on the blood test. It is highly unlikely that the levels of hormone would drop rapidly enough to cause strong positives one day and true negatives the next, but it is theoretically possible.