Answer: rat poison
When Mr. Mentalino stepped outside to make a phone call to Nicholas saying he was going to dump a little rat poison into their Shirley Temple, Harry and Lloyd put a bunch of atomic peppers in his burger. When he came back and took a big bite out of it, it was so hot to him, he flipped back out of his chair onto the floor in pain. He told Harry to get his pills, but Harry, thinking he got his pills, actually got the rat poison and fed them to him, killing him.