Here's an interesting hypothetical scenario I randomly thought of one day:
You are walking alone at night in the middle of a small, lonely shopping center lit only by the faint parking lot lamps. It has been a long day followed by a slow evening with practically no customers at your job toward the end of your shift, and even the shopping plaza outside seems to be deserted now due to the late hour. You've parked your car reasonably far away in a neighboring parking lot due to not being able to find a spot earlier, but it's not a bad night outside and you could probably use the fresh air anyways. However, the route you take to get to your car takes you briefly into an unlit street connecting to the main road, where something unexpected happens that leads to your great crisis.
On your way to your car, you hear the angry shouting of two men from behind one of the nearby restaurants. Looking over some hedges, you spy two employees of a small restaurant arguing near the dumpsters behind the building, one distinctive bald guy in a white chef's jacket and the other a man with a goatee in a black hoodie. At first, you can't tell if they're just being loud and rowdy in a jovial manner or if they're genuinely having a seemingly violent debate over something, but you figure to yourself that it's none of your business either way. Just as you're about to continue along your way though, the man in the white jacket suddenly pulls out a handgun and shoots the other man down with a single shot. Panicking, you back away in the darkness, somehow unseen by the murderer, and retreat to your car where you hastily call the police.
Sometime later, you are questioned by the police over the matter, being the only eyewitness to the event. It's not said outright, but you get the distinct impression that there is somehow a disturbing lack of evidence that directly points to the man in the white jacket as the killer, and that your testimony is THE key factor in proving the man's guilt. No one else had heard the shot, there were no security cameras, and the gun apparently disappeared between the time of the murder and the time the police found the suspect. The suspect's police record is dodgy, having various encounters and previous arrests before on more minor accounts, but nothing serious enough by itself to convince a jury that the man is capable of murder without some proof. Therefore, when the man is arrested on what little evidence there is, it is expected of you to testify in open court as one of the most decisive factors in convicting the suspect. You have no bias or objections in the matter, so you agree to appear in court to let justice have its due.
However, that's when something very unexpected happens. A day or so before the suspect's trial is to begin, you are discreetly approached by the man's disreputable defense attorney, where he gives you an alarming proposition. Apparently, the lawyer has been doing some research on you, and has found out some very dangerous things about your past that he threatens to somehow expose if you testify negatively about his client. What exactly he has found out is up to your own dark imagination; perhaps he has somehow gained hard proof of you doing enough pirating that the fine would be enough to put you in eternal debt for the rest of your life, perhaps he has found out some illegal taboo porn that you are fond of or were in, or perhaps he's found out about some past crime that you could be arrested for and do years of prison for if revealed. Whatever he has found out about you, your life would be utterly destroyed to the point of a living hell if it is revealed.
Herein lies your choice: Would you commit perjury to save yourself from the blackmailer's threats, thereby risking letting a clear and violent murderer possibly walk free, or would you testify truthfully and risk sacrificing your own future to ensure that justice is done? It may be possible that the man is still be found guilty even without your testimony, but it's far from any certainty at all. Similarly, it's possible that the defense attorney may not actually expose you even if you do testify against his client, but again there's no certainty whatsoever. The exact circumstances of the scenario, such as the buildup story or various specifics about the case or what you may be blackmailed with, can be altered around as needed, but the core of the question remains the same: what would you do if pinned between these two choices?