Is it okay for a preschooler to wet his pants once in a while?

Did it happen at school or at home? it might be the case that he did not tell teacher or you in time so that he wet his pant. My daughter tried to tell me and my husband but we did not hear so she wee on her Pyjamas. Once it a while is acceptable but you should tell him how to let teacher or parents know together with Potty training.