Oddly enough, my mom taught computer programming at a community college in the early 80s, and she used the PB&J lesson, though without physically making one. She also used to remind her students: "Computers are stupid. They can only do exactly what you tell them to do, not what you want or need them to do." A series of children's stories, Amelia Bedelia, uses that theme too.

Her first gripe was that the computer didn't recognize the lower case "l" for the numeral "1" ad vice versa. Her old Smith-Corona typewriter didn't have different keys for those characters, or for upper-case "O" and Zero.