We had heard that The Flash was going to feature quite a few "surprise" cameos, and in that respect - depending on how you feel about how these character appearances were utilized - it did not disappoint.
Major spoilers follow.
Aside from some of the Flash and Batman's (Ben Affleck) fellow Justice Leaguers (Wonder Woman, Aquaman, Alfred), most of the characters show up when Barry enters the Speed-Force in the final act. The major cameos have all been well documented at this stage (you'll find a full breakdown here), but there was one that most seem to have missed.
While he only shows up for a millisecond on screen, Digger Harkness, aka Captain Boomerang, can be glimpsed when the camera pans past some alternate Flash designs. Played by Jai Courtney in both Suicide Squad movies (he was killed off in James Gunn's recent requel), Boomerang was one of the Scarlet Speedster's main rogues in the comics, but only encountered the hero for one brief scene in David Ayer's original.
Click here to check out a leaked screenshot
There's a lot going on in the Speed-Force sequence, so there's a good chance Boomerang won't be the only cameo that comes to light as more people go to see the movie.
"Directed by Andy Muschietti, The Flash features Barry Allen traveling back in time in order to change events of the past. But when his attempt to save his family inadvertently alters the future, Barry becomes trapped in a reality in which General Zod has returned, threatening annihilation, and there are no Super Heroes to help. That is, unless Barry can coax a very different Batman out of retirement and rescue an imprisoned Kryptonian…albeit not the one he’s looking for.
Ultimately, to save the world that he is in and return to the future that he knows, Barry’s only hope is to race for his life. But will making the ultimate sacrifice be enough to reset the universe?"
The Flash is produced by Barbara Muschietti and Michael Disco, with a screenplay by Christina Hodson, and a screen story by John Francis Daley & Jonathan Goldstein and Joby Harold, based on characters from DC. Warner Bros. Pictures presents a Double Dream/a Disco Factory production of an Andy Muschietti film.