Every once in a while, the market will open below the point where you would have stopped out your long position or above where you would have covered your short position.
The best practice is to offset the position right away. Bad situations become worse.
Your first loss is your best loss.
Aggravating as it might be, this has not happened enough to me to convince me to stop holding positions overnight or over the weekend. In fact, I'll go so far to say that holding my positions are what I attribute the majority of my gains to.