There are all sorts of medical conditions that require food limitations so chances are good that you do, although you may not know it. Sometimes people are embarrassed to mention their health or perhaps the topic may never have come up.
Sometimes eating "just a little" of a forbidden food can cause a severe reaction, so if a person turns down something you offer please don't pressure them to try it.
It would be a big help for people on diets if everyone would list the ingredients of dishes they bring to potlucks. If a recipe is secret they don't need to be listed in a particular order.
And, if you invite somebody over to share a meal for the first time it's a good idea to ask if there's anything they can't eat and, if there is, to provide a substitute dish.
When you see somebody eating different food than others are having, or a person tells you they're on a special diet, please don't make a big deal of it. While getting used to new diets can be difficult, for people who have been on them for a while that's just the way things are.