The following contains spoilers for Harley Quinn Season 3, Episode 8, "Batman Begins Forever," which debuted Sept. 1 on HBO Max.

Harley Quinn has often made jokes regarding the DC Extended Universe. Some digs have been made at the Snyder Cut and Zack Snyder's toxic fans. There was a reference to David Ayer having his own cut of Suicide Squad hidden in the Batcave. The show's impressive tie-in comic Harley Quinn: The Eat. Bang! Kill Tour poked fun at Jared Leto's performance as Joker.

Season 3, Episode 8, "Batman Begins Forever" took this trend to the next level by addressing one of the DCEU's most sensitive topics. In the process, the HBO Max series almost ruined the dreams of a member of Harley's inner circle. Yet it was worth it to make a witty callback to an infamous big-screen moment.

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Harley Quinn poked fun at the DCEU and Superman's mustache

In "Batman Begins Forever," Harley, Poison Ivy and Clayface reunited with Doctor Psycho and went inside the mind of Bruce Wayne. They wanted to learn the location of Frank the Plant -- not just for their eco-terrorism project, but because Ivy sensed Bruce was experimenting on him. They stumbled across a younger version of Bruce, who kept reliving his parents' murders in Crime Alley in an homage to Batman: The Animated Series, which turns 30 in September 2022.

Clayface used the opportunity for research. After he accidentally got Billy Bob Thornton killed by Catwoman's pet tiger in Season 3, Episode 3, "The 83rd Annual Villy Awards," he assumed the actor's form to work on a James Gunn biopic of Thomas Wayne. He saw the mindscape as an opportunity to learn about his subject. He even tried to interrupt the murder a couple of times to mine what Thomas endured, only to be berated by his peers.

However, the most surprising thing he discovered was that Thomas had a mustache. This put the entire biopic in jeopardy because he/Billy didn't have one -- and when Clayface returned to the real world, he arranged for the mustache to be put on his version of Thomas via CGI. What did this have to do with the DCEU?

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It was a hilarious solution that referenced the fiasco involving Henry Cavill's Superman in Justice League. Per Screen Rant, Cavill was needed for reshoots when Joss Whedon wanted to add more light-hearted scenes to the 2017 movie, but couldn't shave his facial hair because he was already filming Mission: Impossible -- Fallout. Warner Bros. ended up removing Cavill's mustache digitally, which was received badly by both critics and fans.

The mustache situation was so maligned that viewers began doing their own edits online. It didn't really hurt Cavill's profile as the Man of Steel, with the actor potentially reprising the role someday in a future film. But for those who forgot about Superman's terrible CGI, Harley Quinn just produced a clever reminder of that DCEU disaster.

Harley Quinn streams Thursdays on HBO Max.