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Why does shorting a call option have potential for unlimited loss?
You are likely making an assumption that the "Short call" part of the article you refer to isn't making: that you own the underlying stock in the first place. Rather, selling short a call has two primary cases with considerably different risk profiles. When you short-sell (or "write") a call option on a stock, your position can either be: covered, which means you already own the underlying stock and will simply need to deliver it if you are assigned, or else uncovered (or naked), which means you do not own the underlying stock. Writing a covered call can be a relatively conservative trade, while writing a naked call (if your broker were to permit such) can be extremely risky. Consider: With an uncovered position, should you be assigned you will be required to buy the underlying at the prevailing price. This is a very real cost — certainly not an opportunity cost. Look a little further in the article you linked, to the Option strategies section, and you will see the covered call mentioned there. That's the kind of trade you describe in your example.