@attinat It's when someone in a gender bender story fails to grasp that another character is transforming into the opposite sex and instead clings to the idea that the male version and the female version are two separate people despite overwhelming evidence to the contrary, up to and including flat-out ignoring the character transforming right in front of them.
I call it "Kuno Syndrome" after Tatewaki Kuno from Ranma 1/2, who was obsessed with defeating male Ranma and dating female Ranma, and could not seem to grasp the concept that they were both the same person, despite witnessing the transformation happen numerous times (on one occasion, he was physically hugging Ranma during the transformation and failed to notice it).