Change blindness is the phenomenon that occurs when a person viewing a visual scene apparently fails to detect large changes in the scene. I first heard about change blindness several years ago, I think it was on a special explaining magician tricks. It occurs to me that I've never actually seen it, or inattentional blindness, used in a Ranma fan fic even though they really could apply.
In most fics people see Ranma change form and immediately notice. It is rare for a character to explain it away as being impossible (i.e. that was impossible, therefor logically it didn't happen) which I'd expect to be a common reaction to people who happen to see it only once. I'd expect them to say something like "my eyes must be playing tricks on me" or something of that nature.
It's much rarer for a character to not notice when Ranma changes form at all in fan fics. Both change blindness and inattentional blindness could be used to explain as to why another character would not notice Ranma change. Inattentional blindness could be used, in that the character was looking elsewhere at the time and did not see the change occur and therefor would assume that male form Ranma left and female form came in while they were distracted. Change blindness could be used in that they were not paying all that much attention to Ranma in the first place, they registered his presence but not all the details (considering same hair style and color and clothing this could happen very easily). Before anyone says but Ranma's female form has a different hair color, that is not true of the manga (see the colored versions), and the characters in both the manga and anime do not notice the change in hair color (even though we the watchers do).
Video demonstrating change blindness (had embed off by request).
Video demonstrating inattentional blindness:
Videos of similar nature can be found via google or youtube.