This ALWAYS happens to me. Once I maxed out the guests, and like, 5 other random people showed up. I didn't have enough pizza to go around, but luckily the child baked muffins so everyone had some of those too.
Also, I just find it REAL annoying that sometimes I have 2 boxes of pizza if there are tons of family members and guests, and I do the "Call Meal to Everyone". Of course, not everyone gets a slice, so I do it again on the second pizza and some sims end up eating 2-3 slices of pizza, instead of like I want, the sims that didn't get a slice get one. There should be a option that you can have only sims who didn't get a slice get one.
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Silly sims, thinking you can hog all the pizza!