A young couple of our family's acquaintance were expecting a guest for tea, so the wife baked a cake. As was her habit (possibly one inherited from before the invention of self-raising flour), she added bicarb to the mixture.

The guest duly arrived, and was duly fed. They carefully asked how the cake tasted; only after he'd said it tasted fine did they admit that, in going for the bicarb tin, she'd missed, and accidentally used Andrews' Liver Salts instead.