When JB dreams about singing "Master Exploder" in the small bar, he points at one guy in the audience and literally blows his mind. When he starts pointing you can not only see JB's face, but also how his elbow is in a slight angle. In the next shot the camera shows the guy JB is pointing at and his elbow is completely stretched. The views change several times and the angle of his elbow changes after every cut, until the guy's brain is finally blown out.
Actress Amy Adams makes a small cameo during the "Master Exploder" sequence. She is the woman who mouths "I love you!" to Jack Black, and is subsequently covered with the brains of the man whose head explodes.