Under Armour Inc. (UA) - Get Report is having a particularly scary Halloween.

Shares are lower over 19.7% to $11.83 in late-afternoon trading on heavy volume Tuesday, Oct. 31, after the athletic retailer reported disappointing third quarter earnings.

Under Armour served up earnings that declined 58% to $54.2 million and missed FactSet analyst estimates of $84 million. Sales dropped 4.5% to $1.41 billion, which fell short of analyst expectations of $1.48 billion.

The company not only missed revenue estimates, but also reported its first year-over-year revenue decline ever with a 5% drop. Revenue outlooks were slashed to low single-digit rate estimates in the company's earnings statement from an earlier estimate of 11%. Gross margin estimates shrank 220 basis points. Inventory is expected to grow 22% larger to $1.2 billion, while operating income likely won't top $10 million.

While the news was less than welcomed by investors who jumped ship fast, short sellers raked in a killing on Under Armour's earnings flop.

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Short selling interest in the footwear and athletic apparel sectors has been increasing recently, according to a note from S3 Analytics. Shorts on the industry's top10 increased by $787 million in October.

While the entire sector has struggled to catch its breath, Under Armour has consistently been at the back of the pack. Under Armour is the second-largest short in its sector, behind only Nike Inc, (NKE) - Get Report . But it looks like Under Armour could soon pass Nike in the race to the bottom, as short interest in Under Armour jumped by $685 million in October.

As shares of Under Armour toppled Tuesday, short sellers were up an additional $251 million in mark-to-market profits for the day, S3 said. That brings their year-to-date total to $936 million.

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