(CNN) -

Common knowledge when it comes to all things health and medicine might not be so common after all.

Inspired by a topic on Reddit, CNN reached out to health care providers, seeking stories of patients armed with surprising medical misinformation.

The responses ranged from amusing to concerning. We picked a few in hopes that you can learn from their mistakes. Here's what doctors thought they didn't have to tell patients:

Don't stare directly at the sun

"Once I had a 15-year-old patient tell me she couldn't see. ... She had stared at the sun for five minutes straight and burned a hole in the center of both retinas, resulting in a blind spot in the middle of her vision for the rest of her life."

Dr. Cindy Wang of the California Optometric Association thinks this is the only patient she's ever had who didn't know not to look directly at the sun. But Wang has seen other ocular problems arise from sun exposure. Eye diseases, such as macular degeneration and cataracts, can develop due to frequent sun exposure.

"Always wear sunglasses when you're outside," Wang said. "I tell all my patients and still get some that are surprised to hear that."

Sunglasses are the best way to prevent ocular damage by UV rays; for those who don't wear sunglasses, contact lenses or eyeglasses with UV coating provide protection.

'Pulling out' is not a form of protection

"During group session I had one particular patient say, 'You know during intercourse if a guy withdraws before ejaculation, then you cannot contract HIV."

Some of the misinformation about HIV/AIDS can be attributed to fear, said Tasha Kornegay, a sexual health counselor. Many patients are afraid to speak with their doctors about HIV and instead believe the associated myths.

"When discussing and educating people about HIV, we tend to present general information instead of scientific information," Kornegay said. "People may know to wear condoms, but they don't necessarily know why."

HIV can be found in all sexual fluids, including pre-ejaculatory fluid, which is why it's important to always practice safe sex. In order to prevent further transmission, people need to get tested and start dialogues about sexual health.

"Talking can be lifesaving," Kornegay said.

Overdosing on fiber won't cure constipation

"I had a patient come in because of severe constipation, unable to move her bowels for at least a week. She said that the more time that passed without a bowel movement, the more she increased her fiber intake. This 'overdosing' of fiber actually created her constipation issue."

This misunderstanding is common in Dr. Neville Bamji's gastroenterology practice at Mount Sinai Hospital in New York.

"Many people think fiber cures constipation," he said. "It can help with minor constipation, but it isn't meant to be a cure."

Fiber draws water into the intestine and increases stool bulk, but it isn't digestible. The accumulation of too much fiber in the digestive system can lead to side effects such as gas, bloating and abdominal discomfort. Bamji said that it's an old wives' tale that fiber is the way to fix constipation.

If a person is constipated, he or she should try an over-the-counter laxative, which is a little stronger than fiber and made to relieve constipation.

Don't delay lifesaving medical treatment

"I had a professional basketball player who I called with his biopsy of a melanoma, and he told me he was too busy to come for the surgery. Imagine having to explain to someone that they wouldn't be busy for long -- they would just be dead."

Many people don't believe it's possible to die from a mole, said Dr. Debra Jaliman, assistant professor of dermatology at Mount Sinai School of Medicine. But melanoma can quickly spread to major organs.

"Early detection is lifesaving," Jaliman said. "It can be a cure, or it can mean death, should a person choose to ignore their diagnosis."

Surgical removal offers a complete cure to 95 percent of patients.