What is the ruling on someone who declared a person an infidel based on their appearance, and then it became clear that they were not an infidel?

If the description of disbelief (kufr) is applied to a person in a land of disbelief and their Islam is unknown, then there is no harm in that. However, it is more cautious not to rush into judgment. As for in a Muslim land, applying the description of disbelief to a Muslim merely based on their outward appearance is a grave error, unless the appearance is characteristic of disbelievers, such as wearing a cross. In that case, there is no harm in the description based on the apparent. But if one knows of their Islam and accuses them of disbelief, it is a grave sin that requires repentance.