WARNING: The following contains spoilers for "Crisis on Infinite Earths, Part 3," which premiered Tuesday on The CW.

The first three chapters of the Arrowverse crossover "Crisis on Infinite Earths" have spared no expense when it came to cameos, callbacks and Easter eggs. However, there's one Easter egg from "Part 3" that even the most eagle-eyed viewers might have missed.

On Twitter, a fan posted a screenshot of the title card for Earth-666, where Tom Ellis' Lucifer made his surprise cameo. They also pointed out the nod to HBO's Watchmen in the form of a small billboard on the right, to which showrunner Marc Guggenheim said, "Finally. Someone noticed. Special thanks to Damon Lindelof for making this one happen."

RELATED: Crisis On Infinite Earths: Every Earth (and Guest Star) in Part Three

Watchmen and Lucifer aren't the only more mature DC series to factor into "Crisis on Infinite Earths" in some form. In "Part 1," as the red skies began to envelop the Multiverse, viewers were treated to a brief shot of the heroes from DC Universe's Titans. In the case of Watchmen, though, a billboard Easter egg is likely the most we'll see from the HBO series.

After the winter hiatus, "Crisis on Infinite Earths" will conclude on Tuesday, Jan. 14, on Arrow at 8 p.m. ET/PT and on DC's Legends of Tomorrow at 9 p.m. ET/PT.

KEEP READING: Titans Needs a REAL Arrowverse Crossover