In a recent scene, as depicted below, Robin reaches onto a bookshelf full of trophies and other Easter eggs to grab something, but his whole body is pointed directly at an urn filled with "Robin II" ashes (one assumes), per its container. Beside the urn is a crowbar.
This is, of course, a reference to "A Death in the Family," the 1988 comics story in which Jason Todd, the second character to act as Batman's sidekick, Robin, was killed by The Joker.
In the story, Jason was beaten nearly to death with a crowbar and left to die in a room with a bomb. Batman later discovered his body. At the time, there was a 900-number fans could call and pay money to cast a vote as to whether or not Jason should die.
While it was the explosion that technically killed Jason, the crowbar has become a macabre symbol in Joker stories ever since "A Death in the Family," in part because of the brutal and disturbing way the beating was depicted on the page.
That is a pretty disturbing image to have in Teen Titans Go, a decidedly kid-focused show -- and of course, some commenters have brought up the odd timeline issues it potentially introduces since it has long been assumed that the Robin on the show is Dick Grayson, so the notion that Jason Todd would have become Robin, died, and then Dick could have returned to the Titans while still a teen is kind of questionable.
That said, it is just an Easter egg/in-joke, so the likely resolution to that concern is the old Mystery Science Theater 3000 mantra: "Repeat to yourself 'it's just a show, I should really just relax.'"
Other Easter eggs on the trophy wall include Scarface, the Ventriloquist's doll; Renu-Yu, a cosmetic product associated with Clayface in Batman: The Animated Series; and bandages belonging to the Batman villain Hush.
Teen Titans Go airs on Fridays on Cartoon Network.