Laugh, and tell the interviewer that there are challenging experiences as a janitor. The interviewer wants to hear that you are a positive person, and this question is a great way to demonstrate just that. As a janitor, you know what your worst experience has been; the key is to tell the story while smiling and laughing about it. Give the interviewer a brief overview of what happened, and tell the interviewer how you handled the situation. Finally, re-iterate that being a janitor is an important job that you enjoy, and you understand that some yucky experiences come with the territory!