1. During the dragon whisker story arc in the manga, when it flashes back to the time Ranma got the dragon whisker, the Viz version says years ago whereas the Japanese versions says China training period.
2. A recent read Ranma fan fic made mention of Ranma disdaining the use of guns and another one implied that guns did not exist in the Ranmaverse. Personally I think, depending on the situation, Ranma would use guns. He has used drugs, magic, killing techniques, and assorted weapons in the manga, why wouldn't he use guns? (beyond them being illegal and hard to acquire in Japan - not that Ranma actually cares about legalities). As for them being in the manga; Here is a scene from the manga in which Genma, Soun, and Happosai are being shot at:
3. During the hot spring race (in the manga), Ranma makes mention of a prom in the VIZ version, I'm unsure what the Japanese version says but I know that it doesn't say that because the Japanese do not have proms.
4. Something that I've been finding annoying in relatively recently read Ranma fics is the repeatedly revealed investigations of Nabiki Tendo. In the manga, Nabiki does not spend time spying on people to find dirt on them, Nabiki does not have an information network, Nabiki is not good at finding people who do not want to be found (the fiancees are likely better at that than her and if Ranma didn't want her to find him, she never would be able to, if he wanted to he could hide in the same room she is in and she wouldn't know), Nabiki does not show as great an ability to figure things out overly quickly as she does in fanon (Ranma is usually the person who figures out things the quickest -couple examples:what the Ashura's source of power was and who the principals son was), Nabiki does not generally look up information (Akane and Ranma are seen turning to books to find info more often than Nabiki is), etc.
5. Who is the character that spies on people? Quite a few characters, of which Nabiki isn't one of them. If she happens upon something to use against you she will but she doesn't actively seek them out the way other characters do. Ranma does though: During the Sakura-Mochi storyline Ranma wears a tenugui (hand towel used as a sneaking mask in Japanese works) and spies on Akane, during the time Nabiki was his fiancee he clung to the ceiling and listened into Nabiki and Kasumi's conversation, Ranma searches Nabiki's room for stuff to use against her twice in the manga, Ranma also searched: Happosai's room, the Chardon's mansion, Yohyo's mansion, Miss Hinako's apartment, etc. Ranma is far more likely to investigate you and try to find your secrets out to use them against you than Nabiki is.