A little knowledge can be a dangerous thing—especially in the oil market.

Individual investors conditioned by years of (mostly) being rewarded for buying the dips will become acquainted with that proverb as they absorb losses in a popular exchange-traded fund that allowed them to buy virtual barrels of crude. This time there could be collateral damage.

The travails of the United States Oil Fund , ticker USO, in some ways mirror the collapse of inverse volatility note XIV two years ago, but with two key differences: Traders who sold XIV in time had made a fortune, and the repercussions of its 95% collapse were limited to an arcane corner of financial markets.

USO, by contrast, is a perennial money-loser that still somehow controls a whopping 30% of the June benchmark U.S. crude futures contract, according to Goldman Sachs. On Monday, the expiring May contract settled at negative $37 as traders were forced to pay up so that they wouldn’t have to take delivery of actual crude in May.

Storage is nearly full following a crash in demand. Cash still has poured into the fund recently despite that fact. USO already had rolled its positions out of the May contract days ago, but the June contract represents a lot more oil and when it expires there will be even less storage left at the delivery point of Cushing, Okla.