There are some minor things in fanfiction that bother me more than they probably should. One thing is stories in which Ranma, shortly after meeting some new people, tells them his life story. This is very out of character for Ranma to do. The reason this is OOC is because Ranma is a very secretive person and has never told his life story to anyone in the manga. In fact Ranma rarely tells anyone anything about himself if he can avoid doing so, the vast majority of the things we know of Ranma's time before coming to the Tendo dojo is from what other people have told (Genma, Ukyo, etc..).
In many fan made stories, Ranma tells people about his curse shortly after meeting people. This too is out of character for Ranma, though not as much, he generally doesn't tell people about his curse preferring that they don't know. He doesn't really try to hide it but most people who found out about it learned of it from witnessing it in action or learned about it from others. For the few that Ranma showed the curse to, he did not tell them how he came to get the curse or anything about it preferring to let them come to their own conclusions. Though others characters did explain his curse to others. An example from the manga, Ranma did not tell Ukyo about his curse she found about it later and Akane told her about it. He also did not tell Densuke or Yohyo and even stated that he wasn't going to.
Ranma in fanfiction often tells people about his fiancée problems. In the manga he doesn't even tell his fiancée's about each other. Ukyo didn't know about Shampoo and Shampoo didn't know about Ukyo until 7 Volumes of the manga after Ukyo's introduction (Ukyo introduced Volume 9 meets Shampoo for first time Volume 16). So telling complete strangers seems very unlikely to me.
In several fanfics, Ranma will boast about beating so and so or say "you should be scared I defeated Saffron" or tell people what martial art styles he knows. Those are out of character as well. Ranma has told no one what martial arts styles he knows, even the readers don't know how many or what styles he knows. He doesn't even tell anyone about the battles he fought or any of his past accomplishments.
One thing Ranma has told no one about and would actively do everything in his power to hide is his fear of cats. People have and do use it against him in the manga. He also does not complain about any of his problems to anyone, knowing that people may use them against him.
Ranma is very secretive and not an open person which makes it so that we know very little about Ranma. The vast majority of his past is unknown. This is good for fanfics since a fic can be written about Ranma helping the police with detective work when he was twelve or having been an assassin when he was fourteen and nothing contradicts that, though you should probably explain why Ranma doesn’t kill anymore except as a last resort. It also makes it somewhat more difficult since Ranma is unlikely to tell us things about his past since he doesn't volunteer information and would tell you the bare minimum about whatever you asked him (would even lie about it if he didn't want you to know). Meaning if you did make Ranma an assassin at fourteen you'll have to come up with a good reason for Ranma to tell anyone or have someone else tell it.