"No, I wouldn't swap used makeup. [I'm] afraid of eye infections (which I got at a BareMinerals store) or getting a cold sore, which I've never had before." - Karen on Facebook

Experts say: OK, so you didn't heed expert advice and woke up red and swollen. How can you really tell if it's your grody, infected eyeliner or merely a bad reaction to that particular brand of makeup? "An allergic reaction will usually lead to redness, hives and possible swelling," says Ostad. "An infection or anything related to bacteria on the other hand may lead to sores, red spots, blisters and massive swelling on the face."

The verdict: Differentiating what kind of disfigurement or pain you're experiencing can help you track down the culprit, but Hammer warns that sometimes, we don't even need a second party to infect ourselves. "Although products are commonly formulated and tested using effective preservative systems, many different types of bacteria can live in cosmetic products, particularly as the product ages, or is used repeatedly."

The most common include the previously mentioned pseudomonas, staph infections and coliform varieties, as well as many different types of yeast and mold. So, vigorously pumping your mascara wand or leaving your eyeliner cap off as you set it back down on your bathroom counter (guilty) also falls into that "should I be putting this all over my face?" litmus test. And it's safest to assume your fellow swapper is sabotaging their makeup in the same manner.

"How many of you that said no to swapping used makeup still use 'tester' products in the store? How is that any different?" - Stacey on Facebook

Experts say: As I was delving through the digital pages of Makeup Exchange, I came to the same realization as this Facebook fan: Sampling in Sephora is an unofficial, real-life version of the forum -- we're trusting complete strangers to not touch, sneeze or lick those tempting open samples, or to not test a new lipstick when they've got the plague. Disposable Q-tips, sample wands and sample brushes may help, but there's still a risk: Store employees don't vigilantly disinfect after each and every customer, not everyone shares the same concerns about exposure to bacteria (but hopefully more will be, after reading this) ... you might as well order a lipstick that's been used 12 times by a random Redditor.

The verdict: This doesn't mean you have to swear off makeup sampling for good. Unless you're looking for a freebie makeover before your GNO, Hammer reminds us of the main goal for testing out the product: to see if it applies well, feels nice, and matches your coloration. Luckily, there is so much more to you than your face. "Instead of applying the products directly to your eyes or lips, try them out the way we [cosmetic chemists] do: Use a sample wand and apply the products to the back of your hand so you spare your face any possible harm." The next time you're at the makeup counter or at Sephora, use these expert tips to find the perfect color. Then, simply wash the products off when you're done, and you'll leave the store with your eyesight and peace of mind.