Joss Whedon's 2012 entry into the horror genre was typically tongue-in-cheek, filled with intentional, hilarious subversions of various scary movie tropes—including the epic finale, in which a veritable encyclopedia of monsters escape from their cells and annihilate the technicians who've been holding them captive. But while the carnage of that last act was planned, one of its best scenes was the beneficiary of a happy (and, uh, messy) little accident, when the members of the effects crew decided to use their entire remaining supply of fake blood on actor Bradley Whitford's death-by-merman.
For those in need of a refresher, the merman emerges at the tail end of the finale to chow down on the face of poor Steve Hadley (Whitford). It's a bit of poetic justice for Steve, whose greatest unfulfilled wish was to see the merman in action—but its coup de graces comes when Steve's blood sprays rudely out of the creature's blowhole in a sort of aquatic belch.
The effect is hilarious; it's just not what the filmmakers originally planned. With only the one scene to shoot and a huge supply of fake blood being pumped through the creature's head, the merman blew like Old Faithful—and kept on blowing, until the blood ran out and the geyser-like effect was replaced by a farty red mist. Effects master David LeRoy Anderson of AFX later said, "I love the wide shot in the behind-the-scenes video where the blood keeps going and going, because it reminds me of that night, which was just hysterical. I kept thinking, 'How are they going to use this? He's sitting there flapping like a fish, and it looks like a hose spraying blood everywhere!' But the way they cut it together was perfect."