Q. By accident a dairy spoon was used to stir a cup of coffee in a fleishig mug (stone wear) (no milk involved).
The cup was filled with water from a hot water urn and the spoon was put in afterwards.

Both the cup and the spoon were clean and hadn’t been used for a day or more.

A. There is no issue at all, you can use both the mug and the spoon without Kashering.

Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit”a