It turns out Teen Titans Go! #18 was another installment in DC's love affair with chibi characters.
(This didn't scan perfectly, so there are some stretchy-lines.)
The cover, of course, is a lying liar that lies, because Larry doesn't exactly 'join' anything this issue...
One thing you should know about Go! is that just outside the panels, tiny/chibi/super-deformed Titans and villains are always cracking really corny jokes ("What's your favorite game to play, Terra?" "Rock, paper, scissors!"). This time, the first page joke goes to Terra and Larry:
Inside the panels, the Titans respond to a call only to find chibi versions of themselves beating up the bad guy already. They even have their own theme song.
So the regular Titans summon the other chibi Robin,
Nosyarg Kcid "Larry" from Dimension 4 and 9/8ths to try and fix this mess he obviously started somehow. It turns out the chibis were characters in a comic book Larry was reading (hm...). His efforts to put them back in the book just turn the regular Titans into two-dimensional cutouts:
(Is that... a chibi comics writer off-panel?)
The alert is for an army of chibi villains that are now running amok. Which the paper-thin Titans have some trouble handling.
(There's more of them!)
And now, a page I know a lot of people should be able to appreciate better than I can:
Larry finally fixes the Teen Titans and successfully sends the chibi villains and Tween Titans back to their comic book dimension. Only not quite:
Fourth wall: Broken.
This issue says June 2005 on the cover, so when was it that DC had a crossover with the Chibi Justice League that Superman dreamt up after he got high on Kryptonite?
I'm guessing that one came first. Or not.
ETA: Ah, so it dates all the way back to Spyboy/Young Justice. I dug through the tags and found the Superman/Batman: Li'l Leaguers bit actually came after Titans', too. Mightygodking was right: absorbing good ideas from the cartoons is just a proud DC tradition.