### Comment:
You're close but the point of selling a $10C and buying a $15C is so you don't have to buy at market. Each spread has a maximum loss off $500 because if the $10s are executed you also execute your $15 so you don't have to buy at market. 
  
 Basically if someone executes at 10 you have to buy at 15 and take the instant $500 hit per contract. Minus the premiums he received of course.

### REPLY:
Ah, that makes sense. Assuming that guy did exactly that, what was the issue then? Did he lose more than the 500$ due to a margin call?

### END.