Let's say that I sell ounces of gold on SDEX. But it could be anything, bitcoin, oil, corn, fish, strawberries, etc.
I have 100 ounces of gold to sell and the current price of gold is $1300 an ounce.
And the lumen is currently worth 10 cents.
I put a sell offer on the Stellar decentralized exchange offering to sell all of my gold ounces at a price of 13,000 lumens per ounce. After all, the lumen is worth 10 cents.
I go to the movies to see Avengers Endgame. The price of the lumen crashes by 50%.
People see that I'm selling gold at 13000 lumens per ounce of gold. Everyone buys my gold.
I check my phone after the movie and see that I've lost $65,000.