Adaptational Achilles Heel: Downplayed. In the manga, Mustang is only able to incapacitate the Mannequin Soldiers by using his flame alchemy to burn their legs off. Here, he's able to incinerate them completely and reduce them to ash.
Cassandra Truth: Played with. Envy confesses their role in Hughes' murder, but Roy doesn't initially believe them. It's clear Roy wants to believe. But as this is the first time he's actually met Envy, he's having trouble believing that, to paraphrase his words, a moron like this could've taken out his best friend (or alternately, he does believe Envy, but is goading them to get irrevocable confirmation). It's only after Envy transforms into Gracia and Roy understands how they got the drop on Hughes that the Flame Alchemist finally accepts it's the truth.
Curb-Stomp Battle: Deconstructed. Bar none, the most one-sided "fight" against a homunculus in the entire series... except that Mustang is so enraged with Envy that he loses all restraint and his remaining friends worry that he will lose himself to his hatred.
Epic Fail: Envy actually forgot that Mustang killed Lust when they gloated to him about how they murdered Hughes. While they were in the very room where Lust met her end.
Everyone Has Standards: Darius and Jerso, who never met Hughes (as he was long dead by the time Kimblee's Chimeras were introduced), are nonetheless visibly sickened by Envy's remorseless confession and gloating.
Internal Reveal: Roy, Ed, Hawkeye, and the others finally learn who really killed Hughes.
Nice Job Breaking It, Hero: Downplayed example. By pursuing Envy and trying to avenge Hughes, Roy is wasting valuable time the heroes don't have as Father's countdown nears its end. He and Hawkeye also split off from Ed's group to pursue Envy, leaving the heroes' forces divided in the heart of Central Command.
Oh, Crap!: Envy, after they remember several moments too late that Mustang is the one who incinerated Lust to death.
O.O.C. Is Serious Business: When Envy tries to pull his usual Shapeshifter Guilt Trip schtick on Mustang by approaching him as the late Maes Hughes, an enraged Mustang barely hesitates to use his flame alchemy to burn him. Envy sorely wonders how he could burn the very likeness of his late friend. A livid Mustang ominously growls "Maes Hughes is dead..." This drives home how Mustang feels such hatred and rage towards Envy that he'd willingly burn the very memory of the fallen comrade he grieved not months ago.
Mustang:That's enough! You were stupid enough to confess, and even more stupid to boast. Everything you've said is fuel on your funeral pyre. So then, I think I'll begin — by burning out your tongue!