How do you get a false positive on a pregnancy test?

Many reasons.: There are many ways of getting false positives from this. The test kit can be too old and the reagents expired, the test can be done wrong/applied wrong, false evaporation lines can appear if read too late after application, and a woman may have received an injection of HCG for infertility treatment causing a false positive to appear. Finally, tunors can produce HCG and cause a false positive.

Yes: Not as common as false negatives but they do occur.