SPX closed at 2721.33 at 1 pm (PT) today (5/25), so my short call was only $1.33 in-the-money. However, right before the closing bell, the "bid" was $1.85 and the "ask" was a whopping $2.90!!!! UNBELIEVABLE!!!
So .... instead of taking a larger-than-necessary loss by buying back my ITM short call, I simply let the option expire in-the-money (pretty unnerving, I must admit).
Since I collected $1.10 when I sold (to open) the call and it ended up $1.33 ITM, my loss will be around $23 rather than $180 (if I had bought back the call).
The trading rules for "index" options (SPX, RUT, VIX, etc) and the trading rules for "equity" options (SPY, IWM, etc.) are VERY DIFFERENT. Please make sure you thoroughly understand these rules before trading with real money.