Stephany Aulenback tried out a recipe for "Chemical Apple Pie," a beloved science experiment that uses cream of tartar (potassium bitartrate) to trick the human tongue into tasting apples, though no apples are, in fact, used in the pie. The pie tasted pretty good, and Aulenback unearthed a lot of interesting history of the dish:
Further searching revealed that it is an even older recipe than that, dating back at least to the mid 1800s. Recipes for it have been found in the Confederate Receipt Book in 1863 and Mrs. B. C. Whiting's How We Cook In Los Angeles (1894) in which she referred to it as "California Pioneer Apple Pie, 1852" (if you follow that link, choose Mock Apple Pie from the menu on left). It's certainly easy to imagine that, historically, apples were difficult to come by out of season, at the end of a long journey across the prairies, or in an poorly supplied army camp. (As Mrs. Whiting is quoted saying, "The deception was most complete and readily accepted. Apples at this early date were a dollar a pound, and we young people all craved a piece of Mother's apple pie to appease our homesick feelings.") Presumably crackers—or the cracker-like foods of the time—kept better, and one sometimes needed to dream up new, more interesting ways to force oneself to ingest them yet again.
Apparently, the Nabisco company appropriated the recipe in the 1935 when they printed it on the boxes of their fancy new Ritz crackers; today, most people who are familiar with Mock Apple Pie associate it with Ritz. You can still find the recipe on their site, where they warn you to watch your serving size. Probably because of the calorie and fat content, not the muscle toxin.