Leakage of semens was the first piece of information that I came to know about the harmful effects by excessive masturbation. What is leakage? When you are in mode, you will notice that the tip gets wet. It could also happen if you are really happy and excited (which might have some sexual kick in it). You do not have to be fully aroused for this to happen. It happens when you are limp but most importantly it happens you are fully erect also.
I believe this is happens because the penis muscles gets weak due to excessive masturbation and it can not stop the semen from stopping. An other explanation would be, you body develops this automatically when you practice excessive masturbation.
This I believe is the prelude of problem associated with excessive masturbation. It may happen to individual who has not practiced excessive masturbation. Our research is limited on this topic.