No matter how careful you are in concealing your weapon it is simply impossible to ensure that you will completely conceal in all circumstances and at all times.
What if you trip and fall, causing your jacket to fling up and accidentally revealing the weapon? It doesn't mean you "improperly" concealed or that you are "giving a bad name" to gun owners.
I'll openly grant you that a careful CC licensee should not let it happen often and should take steps to ensure that it doesn't happen often. But it is unreasonable to fault him for the one time that somebody else catches a passing glimpse of the gun under his shirt. And it certainly doesn't merit calling the cops on him and thereby leading to his execution in broad daylight.
I saw an off-duty police officer’s concealed carry gun yesterday at a Boy Scout gathering. He’s good at concealing, but he was lifting something up into a truck, which exposed the barrel end of his concealed carry holster. Happens all the time, even to the pros.