Let's say it's for something petty-ish. Like they grabbed the last mango at the supermarket that you had your eye on and were walking over to get it. Idk you're the witch so you can choose your level of vindictiveness. Explain what they did if it's fitting in with the curse.
For grabbing the last mango at the supermarket? I'd just make their chicken penne pasta come with no chicken. For something more serious I'd flip gravity upside down for them.