For those talking about false postivies, they are not impossible, just rare. There could be various reasons for a false positive test.
1) Chemical Pregnancy
A chemical pregnancy is basically an early miscarriage. Research has shown that half of all first pregnancies end in miscarriage. Due to the high sensitivity of home pregnancy tests, many women catch early pregnancies but end up going on to have their periods due to a chemical pregnancy.
2) Missed Reaction Time
For accuracy in home pregnancy tests it is essential that you read the test results in the stipulated reaction time. If you let the test sit for too long (after the instructions on the box tell you), the test is invalid. This may cause it to show false positive results.
3) Chemical Interference
Drugs that have the pregnancy hormone Human Chorionic Gonadotropin (hCG) in them can cause incorrect results in home pregnancy tests. The Human Chorionic Gonadotropin (hCG) in them may be responsible for the false positive pregnancy test. If you have had an HCG shot due to infertilty treatments, you need to be sure the shot is out of your system before you attempt to use early pregnancy tests.
4) Evaporation Lines
While technically not a true pregnancy, some pregnancy tests leave a faint evaporation line as the urine crosses the test. This faint very light grey evaporation line in the test area is often interpretted as a positive result.