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

Well it depends on two things, firstly is it a one off incident and secondly what stage of potty training they are at currently. If it is a one off incident, try not to embarrass your child about it especially if they are still under going potty training. They might still be unsure as to how long they can control themselves. If this has happened too many times before, you might want to consult a doctor to find out if there are any serious issues.