Ok so in my case where I forgot and went short out of accident it wasn't illegal I guess. I do sell puts at times for some securities and do it on margin without having the full cover. However I do assume that I will buy it back and not let it get exercised. Now with the american style options the real risk is that someone exercises them before expiry meaning that I end up with the stock and margin call. I don't sell naked calls however, that's far far too risky though I have contemplated selling a straddle in case of some stocks during ER to benefit from the high IV and the imminent collapse the next day (assuming the stock doesn't really move as much). How is that considered? Let's assume some stock like MSFT has earnings on Thursday and I sell the ATM straddle. I guess if I just buy it back on Friday for profit/loss it's fine, but if someone decides to exercise their call or put in after hours post-ER, then I'm in deep ****?