Quote from my7tvette:
With RUT ATM calendar spreads, what are the potential risks of holding the spread into expiration (let's say you are long a call and put calendar) and then selling your long options that Friday?
The biggest problem is the way the settlement value is calculated, which essentially means that it has very little to do with the index value on expiration Friday. In other words, up until expiration Friday the two options have the same underlying, on expiration Friday the underlying is no longer the same and you can end up with an unexpected loss.