When my Assistant Editor position called for me to proof, I had to keep my eye open for many different types of problems, ones which don't turn up when dealing with straight text.
Accidental typographical errors made by a letterer. Costume mistakes drawn by an artist (perhaps the webbing in Spider-Man's armpits was penciled or inked incorrectly, or the wings on the Submariner's ankles were drawn too high on the leg). Inaccurate references by the writer in those editorial boxes at the bottom of panels which would point back to earlier issues. Or even something deliberate that one of these creators was trying to get by the editorial department and sneak into print as a joke that wouldn't make the company laugh.
On the very last page of the issue, I did indeed find something which needed changing. (It might help if you click on the image above once, then again, to see the original artwork at its largest size.) Looking at the excerpt from an unidentified issue of Luke Cage, Hero for Hire (unidentified because, though I may I have kept a photocopy of the panel, I don't keep track of everything, so it will be up to someone in the blogosphere to track down the issue number), can you spot the faux pas that I needed to get changed?