Examples of opportunity loss

Suppose you have a situation where you take an entry signal only for the position to go nowhere, so you end up getting frustrated and close the trade. The next thing you know, the stock takes off in the direction you were attempting to trade, leaving you left on the sidelines. What have you lost? 

Some people would say that, as you haven't lost anything, you should simply move on and concentrate on what is happening in front of you now.

While I agree with the fact that you need to move on, I would  disagree with the point about not having lost anything - there is an opportunity loss incurred.