A case of mucosal burn during the placement of fissure sealant on the first permanent molars of a 9-year-old Malay boy is presented. The erythematous lesion with accompanying burning sensation appeared a few minutes after the etching liquid, containing 37% by weight phosphoric acid, had accidentally come into contact with the buccal mucosa on the right side of the angle of the mouth. The mucosa showed complete healing after one week. The use of rubber dam for tooth isolation while doing fissure sealant is essential to avoid accidental contact of potentially caustic chemicals, such as the phosphoric acid etchant, with the oral mucosa as it can result in mucosal burns.