I was reading a charming story for children about some little mice when - wham! suddenly I was jerked out of the fantasy. The mice were walking paw in paw. Now, up until that point, I was enjoying the story and not really thinking about the level of anthropomorphism. I accepted that the mice were living a human lifestyle, with mousy additions. But that one phrase took me into reality - if the mice were walking paw in paw they had to be up on their hind legs, and that created a visual picture that jarred with my vague imaginings. Worse, once I'd been taken out of the story, the rest of the fantasy was undermined.
We can accept all sorts of things as being real within a story, from the wizarding world of Harry Potter to the magical realism of Gabriel Garcia Marquez. But the second something challenges the reality of the story world, the whole thing falls apart.
All fiction is a fantasy. If a character behaves in a way nobody would in real life, the fantasy is exposed. How many single women, on returning to their flat, start to take off their clothes without turning on any lights? According to a certain type of film, this happens all the time, but when I see it any tension dissipates as I stop suspending disbelief. In a romance, I'm quite happy for the central couple to bicker their way through the first three quarters of the book before realising they're in love, but the change has to be gradual. If it's bicker, bicker, bicker, oh look we're in love! you've lost me.
Implausibility takes the reader out of the story world more than anything else, but any hiccup in the reader experience is to be avoided. That's why, if someone says they didn't get something your reaction shouldn't be to defend your work but find out exactly what it was that made them leave the story world. Because if they leave, you've lost them.