Unexpected Positive

Someone takes a medical exam for some reason other than genuinely wondering about his/her health. Surprisingly, the test is positive. (Meaning they have whatever condition they were tested for.) Depending on the show and the condition, this can be Played for Laughs or Played for Drama. In the latter case, it may be a Lethal Diagnosis.

Examples of Unexpected Positive include:

Film

In Baby Mama Angie's pregnancy ruse gets her into the obstetrician's office, and she discovers that she actually is pregnant.
Played for tragedy in Kids. A character who has only had sex once has an HIV test to encourage her promiscuous friend to do the same. Guess which one's positive.

Live Action Televsion

The Brady Bunch: Carol lets the doctor check her tonsils to show Cindy that it won't hurt. Turns out Carol and Cindy both need their tonsils out.
The Golden Girls: Dorothy takes a hearing test to show Sophia it's no big deal. Sophia does not need a hearing aid; Dorothy does.
Growing Pains: Maggie is doing a series of news reports about regular medical checkups, and thus undergoes all the usual tests. This is how she discovers she's pregnant.
- In another episode, Jason allows a pediatrician to examine him to show a teenage boy it's no big deal. This is how his hernia is discovered.
In Scrubs, there was an episode with a hypochondriac patient who demanded every test available. Eventually, as they were running out of tests and trying to convince the guy he was obviously perfectly healthy, Dr. Cox sets him up with an extremely uncomfortable test for a one-in-a-million condition in the hopes that he'll go away. It unexpectedly turned out positive.
In Coupling, Sally buys three pregnancy tests because she doesn't know what a negative test looks like and asks Susan and Jane to take one each. Of the three of them, it's the apparently infertile Susan who turns out to be pregnant.
In That's So Raven, Raven and Corey go to the dentist. Corey's scared, so (after having a vision in which one of the pair is shown to need a filling) Raven gets in the chair to show him that it's not so bad. Guess who needs the filling?
- The exact same scenario was done years earlier on The Jack Benny Program. Jack goes with a neighborhood boy to the dentist because the boy is scared. Guess which one needs a tooth pulled?
An episode of ER had Carter demonstrating to several medical students how to examine a patient's lymph nodes—only to discover a lump in the neck of the student he was examining. After doing a biopsy, he gently informed him that he had lymphoma.
During an episode of Doogie Howser, M.D., the senior Dr. Howser flashed back to a time when he was examining his son, who had asked him to demonstrate to him. Upon examining his abdomen, the boy yelped in pain, and the horrified Dr. Howser realized he'd just discovered that his son had leukemia.

Real Life

Truth in Television: John Holmes got an AIDS test to encourage other porn stars to do so. He tested positive.
Truth in Television: A reporter for the Australian 60 Minutes had this happen during a story on cancer.