Suicide Squad: 30 Easter Eggs & References You Must See
21. The Flash's Cameo
Captain Boomerang (though they drop his official title, presumably because it would make him sound too stupid or something) has a backstory that involves him robbing a diamond exchange in Australia with an accomplice and then turning on him.
Just as he thinks he's about to get away, Ezra Miller's Flash turns up, scoffs at him with a barb about "honour among thieves" and takes him down.
Traditionally, Boomerang has been a Flash villain rather than a Batman one, so the cameo - however brief - fits.