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I feel like this is missing an all-important caveat: What happens if you don't finish what you ordered?
I didn't think about that, but I already have a good idea for that case.
Mixing together ice could have the obvious problem of mixing up the wishes in some way. Sometimes exchanging parts of one wish with another and getting hilarious results.
One person eating the ice cream that another person's wish generated can have strange effects if the Person has different tastes and desires then the wisher. It gets even worse if People share an ice.
Finally, not eating an ice, either leaving a part of it behind or just not touching it is very bad. It destabilizes the wish and leaves it without boundaries. You can never know how such a wish is fulfilled and it may even go out of control, start doing things that were not part of the original wish though it would still resemble the original version. Like, instead of making your neighbour's breasts grow, it becomes some Kind of Virus like effect that makes breasts grow anywhere your neighbour touches. The severity of the instability would depend on how much ice is left.