In Green Lantern #49 (December 1966), the titular hero thinks that Henry Peters, a special effects technician for The Dazzler, a popular TV show, may be committing crimes while dressed as that show's supervillainous title character. So he decides to scan Henry's mind (holy invasion of privacy, Batman!) but discovers that Peters' mind is shielded somehow. So Green Lantern decides to chase the Peters, understandably spooking him. If a guy in spandex started chasing me with a glowing green hand, I'd run too.
The poor guy barks his shin on a concrete block some idiot left in the middle of the sidewalk and then smacks face-first into a birdbath or water fountain with what appears to be enough force to break his jaw and smash a few teeth.
"Too bad," is Green Lantern's compassionate response. His "idea" is to scan the guy's brain again while he's unconscious, figuring that his mental defences might not work while he's out.
It works, much to the outrage of the American Civil Liberties Association, no doubt. Green Lantern quickly learns that the man has mental powers because he's from another planet, a representative of an advanced and peaceful culture. The interstellar ambassador, it turns out, is quite taken with the aliean Earth concept of drama and decided that he wanted to be in show biz. He got a job in the industry, doing technical work for the sheer love of it. His unconscious mind reveals that he was about to seek out Green Lantern to warn him about The Dazzler's next crime, and to share his identity.
But Peters starts to wake up before Green Lantern's scans can reveal who the real bad guy is. So the stalwart hero, even though he now knows for certain that this man is innocent, even though he now knows that Peters wants to share his information willingly, reacts thusly:
Yes, that's right - he punches the guy in the jaw to keep him unconscious, just so he can keep scanning his brain. Nor does he ever offer an apology, or even think to himself that maybe, just maybe, he crossed an ethical line here.