I got one:
My friend was at a conceal/carry class, and the instructor was discussing various situations in which it is permissible to shoot someone in defense of another.
The scenario was this:
"You walk into a gas station, and see a man jump over the counter, and attack the clerk. The man pulls out a gun and orders the clerk to the floor. The clerk screams for help. What do you do?"
Most people in the class apparently responded that, in those circumstances, it would be permissible to shoot the attacker.
But the detail the instructor deliberately left out changed everything: The "attacker" was a undercover police officer trying to make an arrest on the clerk.
Anyone who would have shot in good faith would have shot a police officer.
One student replied, "Well, the cop should have announced who he was."
The instructor replied, "Those were not the circumstances, and whatever the cop could or should have done, you just killed the good guy, and regardless of what happens at trial, you will have to carry that around for the rest of your life."
Another example of "not everything is as it might appear."