Margarine made from vegetable oils (non-dairy) looks just like butter, but is normally labeled as parve.
Shouldn't it be considered chalavi according to the principle of mar'it ayin?
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If it was an issue of Mar'it Ayin, it would only require clearly indicating that this was margarine and not butter. However Mar'it Ayin only applies in this type of case to a situation when the application is unusual. When everyone will think of margarine when they see it, it isn't a problem. Rabbi Chanoch Dov Padwa makes that point explicitly about margarine in חשב האפוד 1:20.
When Kosher margarine first came out, people who remember told me that Rabbis required leaving the packaging on the table while eating it for Mar'it Ayin reasons. But after margarine become commonplace that practice was discontinued.