Do you have the scourge of zero hour contracts in the US? Basically a person can be contractually obligated to work when demanded but with no guarantees of minimum hours or salary.
I don't think we call it that, but I don't see much diff between "zero hour contracts" and the random shift scheduling others have mentioned. One week you have 25 hours, the next week you have 5, but you don't know and can't plan around it. There may be a technical/legal distinction, but the practicality of it seems the same.