SPOILER WARNING: This article contains major spoilers for "Inside Voices," the latest episode of Agents of S.H.I.E.L.D.

Agents of S.H.I.E.L.D. just dropped a major reference to Dr. Horrible's Sing-Along Blog. In "Inside Voices," the latest episode, S.H.I.E.L.D.'s own Phil Coulson mentioned the Evil League of Evil.

Under the influence of gravitonium, Carl "Absorbing Man" Creel went to confront Coulson. However, Coulson thought Creel was there to convince him to join General Hale's team of supervillains. "You can tell the General there's nothing you can do to make me join her Evil League of Evil," he said, obviously referring to Hale's pseudo-Thunderbolts team.

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In Dr. Horrible's Sing-Along Blog, the title character's greatest ambition is to join the Evil League of Evil -- a goal which he accomplishes, at the expense of his love interest Penny's life. At the end of the superhero musical dramedy, he joins Bad Horse, Snake Bite, Fake Thomas Jefferson, Tie-Die, Professor Normal, Fury Leika and Dead Bowie on the League.

Of course, both Jed Whedon and Maurissa Tancharoen had a big hand in bringing Dr. Horrible to life alongside Joss and Zack Whedon. The two Agents of S.H.I.E.L.D. executive producers wrote and cameoed in the musical. Whedon appeared as both a member of the Bad Horse Chorus and Evil League of Evil member Dead Bowie, while Tancharoen played Captain Hammer Groupie #1. If anything, it's surprising they waited this long to reference Dr. Horrible on the Marvel series.

Airing Fridays at 9 p.m. ET/PT on ABC, Marvel’s Agents of S.H.I.E.L.D. Season 5 stars Clark Gregg, Ming-Na Wen, Chloe Bennet, Henry Simmons, Ian De Caestecker, Natalia Cordova-Buckley and Elizabeth Henstridge.