Due to the nature and pricing of box spreads (and similar strategies), they can be especially susceptible to the risks associated with an early assignment. The following links explain what a box spread is and why a short option would be assigned early. 

All of these potential reasons for early assignment get priced into the options. Because of this, it can be possible to sell a box spread (or similar risk-free strategies) for more than the width of the spread. This gives the illusion of risk-free profits, but early assignment and the associated costs could wind up costing much more than the additional credit received.