One pet peeve of mine is ice cream being measured by volume. Similar dairy products, such as yogurt, sour cream, and cottage cheese all go by weight. The problem is that cheap ice cream is full of air. The volume is irrelevant unless you know what percentage of that volume is air.

Another is food packaging listing the calories by serving size, which is a completely arbitrary amount. So if I want to know how many calories I just consumed, I first have to do arithmetic to figure out how many calories are in the full package, and then do another calculation based on how much I ate.

Neither of these things are illegal, however. They are cheap tricks intended to make it harder to figure out wtf it is you're actually buying and eating.