What types of things could return false positives in std testing?

Lupus, old age...: False-positive syphilis screens are common in older folks and especially in lupus; that's why we follow up. The herpes antibody blood tests are notoriously unreliable. Interpreting microbiology smears requires expertise; i once had to defend a bogus "child abuse" case in which a pediatrician mistook chickenpox for gonorrhea based on his ignorance interpreting a lab test.