1 Sam 24:3-21 / Mark 3:13-19
The phrase "to be caught with your pants down" means that you are exposed in an embarrassing situation. It can also mean that you are caught unprepared for a situation or an event.
Certainly no one would like to be caught in a situation like this. Moreover it may not be something of your own doing or that it's consequence of your earlier actions.
It is more like you walked unknowingly and unsuspectingly into a situation and got caught with your pants down.
In a way that was what happened to Saul in the 1st reading. He was on the hunt for David, and along the way he came by a cave and went in to cover his feet, ie. to answer the call of nature.
Unknowingly and unsuspectingly, he actually walked into the very cave that David and his men were hiding.
Saul was caught with his pants down, literally, and he could also have been killed with his pants down. But David spared him because of his fear of the Lord and his respect for the Lord's anointed.
This incident also showed the true character of David - he would not take advantage of a man in his most vulnerable moment, nor would he disregard the law of Lord even for the sake of his own life.
In the gospel we also see the true character of Jesus as He sent His disciples to preach the Good News; He also gave them the power to cast out devils.
He won't let His disciples be caught in a vulnerable situation nor be exposed to dangers that are beyond their abilities to handle.
Neither would Jesus let us be caught in that kind of situation. Yet we also need to learn from David. Let us not take advantage of a person in his most vulnerable moment and let us keep the Law of the Lord and protect the dignity of others especially when they are caught and exposed in an embarrassing situation.