Assignment, Exercise, and the Early-Exercise Surprise

American versus European exercise. Almost every option a retail investor trades on a single US stock is an American-style option — the buyer can exercise at any moment up to and including the expiration date. Most US index options (SPX, NDX, RUT and a few others) are European-style — exercise can happen only at expiration, never before. The exercise style is printed on the contract specification at the exchange and is not a detail the trader can negotiate.

Early exercise on an American call is almost always a dividend story. The classic trigger: a deep-in-the-money call whose remaining time value is smaller than the upcoming dividend. The holder is better off exercising the call (capturing the dividend as a shareholder) than holding to expiration (where the dividend payment would be lost to whoever owns the shares on the ex-dividend date). For the short side, this means a deep-ITM short call on a dividend-paying stock just before ex-dividend is a real assignment-risk trade — not 'unlikely.' Calendar this risk explicitly on every covered call you write through an ex-dividend date.

Two surprises that bite retail traders. First, even an OTM option at the Friday close can become ITM in after-hours news and still auto-exercise — most brokers run an automatic exercise threshold ($0.01 ITM by default) and a Friday-night earnings beat can flip a contract you assumed was dead. Second, assignment on a short call you wrote in a taxable account can trigger a long-deferred capital gain you were not planning to realize this tax year. Both of these are mechanical, both are well-documented at the broker, and both routinely catch the trader who treated assignment as a remote abstraction.