If you've never seen "A Nightmare on Elm Street," Glen's grisly death comes just after Nancy discovers a vital bit of information about their nocturnal tormenter. Unfortunately, she's unable to reach Glen in time to save him. And once the doomed lad nods off, Freddy promptly pulls him into his own mattress, with a ceiling-spraying geyser of blood erupting from the resulting cavern. For point of reference, according to Craven's own 2014 tweet, that geyser required upwards of 500 gallons of fake blood.
According to Rolling Stone's 2014 behind-the-scenes look at Glen's "A Nightmare on Elm Street" death scene, it seems creating that geyser almost led to an on-set disaster. And per that RS article, the set used for the shot was the same rotating beast built to shoot the death scene of Amanda Wyss' Tina, another of the film's most iconic kills. In order to create the effect of the blood spewing from the mattress, Craven and crew used the rotating set to shoot the scene inverted. That means the blood is actually flowing down out of the mattress instead of up from within, with the plan being to control the flow by subtly rotating the set.
Things did not go according to plan. And unfortunately the crew had failed to insulate the countless exposed electrical wires powering every device on set, which proved a potentially deadly oversight.
