Well, obviously somebody saw. Nobody clocked it right away, that's probably more accurate.
Although most people have been pouring over the finer details of our first look at Justice League, a few people noticed something in the background during the scene where Barry Allen AKA The Flash meets Bruce Wayne AKA Batman for the first time.
Here's a screenshot of the scene in question.
Computer, zoom in to bottom-right quadrant and magnify.
Needless to say, Rick & Morty's fanbase - not to mention its creator, Dan Harmon - got wind of this and promptly went nuts.
— (((Dan Harmon))) (@danharmon) July 23, 2016
What's interesting about this, other than the fact that Barry Allen / The Flash has excellent taste in TV shows, is it really does underline just how much of a departure there is from Batman v Superman's tone. You could really see in the short bit of footage we all saw that Justice League was much more lighter, much more fun than Batman v Superman.
Bruce Wayne was grinning, Wonder Woman was being a bit sarcastic, Aquaman's some kind of high-functioning alcoholic and Barry Allen's talking about his second-favourite chair. Could you imagine any of this in Batman v Superman: Dawn of Justice? Absolutely not.