If you weren't racing out to buy the Stranger Things soundtrack on cassette, you might not have realized that cassette sales are growing. In 2016, sales jumped a whopping 74 percent, and they've continued to climb. In 2018, sales increased another 23 percent, reaching a total of 219,000 tapes, compared to 74,000 in 2015.

Apparently, there's only one factory refining gamma ferric oxide, an essential ingredient used to make the magnetic recording tape. According to NAC, that facility has been "under renovation" for most of the year, and NAC is waiting on 50 tons of back-ordered gamma ferric oxide.

Fortunately, this doesn't look like a long-term problem. NAC should receive at least 11 tons of ferric oxide this month, and after that, it expects more regular deliveries. But this hangup does reveal a hidden cassette weakness: the whole cassette industry seems to rely one one gamma ferric oxide refining factory.