The glass in the tank breaking at the beginning of the film was actually a happy accident. You see the heat from the melting wax corpse was too much for the glass to bear and it cracked. That was in the end a good thing, it at least gave us an explaination for how a horribly rotten corpse could escape from a tank designed by the U.S. Army's core of engineers, a supposedly unbreakable tank.
Originally the movie was to be directed by Tobe Hooper (The Texas Chainsaw Massacre, Poltergeist) and produced by George Romero, the original creator of the Living Dead series, but Living Dead producer, John Russo, convinced them to end their involvement and even tried to sue the filmmakers for using "Living Dead" in the title. Ironically, the script is adapted from a novel by Russo.
Don Calfa, who plays Ernie, got annoyed with the ripped bottom of his sweatsuit flapping against his leg in one of the scenes so he cut it off with scissors, which was unscripted, yet kept in the film.