Red M&M's disappeared in the '80s after a study linked a certain red food dye to cancer, despite Mars not actually using that dye.

If you’ve watched the new season of “Stranger Things” on Netflix this week, you might have picked up on a tiny gaffe: Red M&M’s did not exist during most of the 1980s, but they do show up in the show, which takes place during the early part of that decade.

A minuscule detail, perhaps, but it raises the question: Why was the decade void of red M&M’s until 1987?

M&M’s are colored with Red 40, an artificial food dye approved by the Federal Drug Administration. In 1971, a Russian study claimed that Red No. 2, another artificial food dye, was carcinogenic. Although it was never actually proven, the FDA banned the use of the dye in 1976. Even though Mars Inc. didn’t use Red No. 2, it decided to pull their red M&M’s from the market the same year to avoid consumer confusion.

Red M&M’s didn’t exist for more than a decade before Paul Hethmon, a college student in Tennessee, began a viral campaign to convince Mars to bring back red M&M’s. What started as a joke — Hethmon sent an invitation to friends to join “The Society for the Restoration and Preservation of Red M&M’s” — turned into a movement. First, his college newspaper wrote about the Red M&M society, and then the story got picked up by national publications.

The red M&M was reintroduced in 1987. Mars sent Hethmon 50 pounds of red M&M’s in celebration.

Red food dyes have long been subject to controversy, and that continues to be the case to this day. Although Red 40 and Red No. 2 are artificial food dyes, there also are naturally occurring food colorings exempt from FDA oversight that are commonly used. These include carmine, which is derived from the cochineal insect. It’s also sometimes listed as E120 in the ingredients list on packaging.

The cochineal insect is found in South America and Mexico. It is crushed to obtain a bright crimson dye, and its use dates back centuries. It takes about 70,000 insects to make one pound of the dye.

Other FDA-exempt red food colorings include beet power, beta-carotene, paprika and annatto extract.