It was July10 expiry yesterday and BHP closed at $40.46. One would think that $40.50 short calls would be safe just as the OP thought his BHP otm puts were safe. Not so according to the list of this morning's assignments inserted below:
One thing I have found very annoying is that you can be assigned on just one lot of your position. Most Oz brokers charge a minimum fee for everything so you get charged this full amount for a one lot. In many cases, it would not cost any more to be assigned ten lots.
It makes me wonder just how random this is. IMO, it begs the question if brokers just "randomly" allocate one lot each to several retail traders as this would bring in significantly more fees. Yes, I am a skeptic... lol