Warning! Spoilers for the Attack on Titan manga below!
Attack on Titan's creator is known for often slipping in Easter eggs and humorous jokes into the series' art despite the bleak content, and the latest one uncovered by fans sure is Super.
In chapter 71 of the Attack on Titan manga, one Titan is devouring someone while holding to fingers to its forehead much like how Goku does when he uses the Instant Transmission technique in Dragon Ball Z.
This happens during the "Return to Shinganshina" arc of the series where Keith is telling Eren all about his father Grisha. Grisha had appeared within the walls while suffering from amnesia, so Keith decided to help him get reacquainted with life within the walls. Grisha was such a good doctor, he eventually cured a plague and fell in love with Eren's mother much to the chagrin of Keith. The Dragon Ball Z moment happens when Keith describes his many failing missions as a commander before eventually turning the service over to Erwin.
As mentioned earlier, this isn't the first time Isayama had inserted a goofy reference to the series. He once drew the cast of HBO's Game of Thrones into a crowd of Titans with their likenesses taking on ghastly forms.
For those unfamiliar with Attack on Titan, the series was originally created by Hajime Isayama. Starting its run in Kondansha's Bessatsu Shonen Magazine in 2009, the series has been collected into 23 volumes as of 2017. It's set in a world where the last remnants of humanity live within a walled city in order to escape the danger of the Titans, a race of giants monsters that eats humans. Eren Yeager joins the military with his two childhood friends Mikasa and Armin after the Titans break through the wall and attack his hometown.
The series was adapted into an anime from Wit Studio and Production I.G. with a third season premiering in 2018. 68 million copies of the manga have been sold as of 2017, and has been met with major critical and commercial success worldwide.