Sometimes the risk isn't always that clear. Especially not for beginners that trade spreads. They think the value of the spread is the risk, but obviously there's more than that.
A few months after I started trading I lost $15k because I traded 50 calendar spreads, for a total debit of $500, where I got assigned on the short leg (even though expiration was still 6 months out!!). I thought my risk was only $500, but god was I wrong.
Due to the assignment my buying power got very negative and my broker auto-liquidated all stock directly the next morning straight at the open + some of my other positions.
Although my short stock was fully covered by the long options, with no delta at all, theoretically there was no risk, but due to the risk management policy & algo's of my broker my whole account got messed up. They didn't liquidate the long options, so now the delta on this position was huge. I tried to close and repair everything asap, but with the wide markets at the open and stock moving in unfavorable direction, this joke cost me quite a bit.
Had they not auto-liquidated everything directly and given me an hour to resolve the issue, it would have all been fine though. Just an example to illustrate how newer traders can blow up without them realizing they have a risky position on.
