I have a 5 year old potty-trained son and I have found that I have to clean this one area more often than ever before. The back of the base of the toilet gets very messy and needs to be cleaned every couple days.
Young boys get distracted very easily. They may be aiming at the toilet one second and the next second something catches their eye. If you have a young boy who is potty trained, make sure you check around the base of the toilet (especially around the back) for any mishaps!