I think sometimes writers have a plot in mind and the character is required to do things for the plot that don't make sense in the context of what's been previously established by the author.

An example here would be Melrose Plant in Martha Grimes' books. In one of the earlier books in the series, we tap into one of his childhood memories of being on a fox hunt, smeared with the blood at the end-- and horrifically traumatized, etc., etc. One of the worst experiences of his childhood.

In a later book, the plot she had set up involved him hunting-- and it was all jolly fun and this memory which was so significant to him in a previous book didn't come up at all. (I could understand him making a conscious effort to get beyond it, or realizing as an adult that he'd overreacted... or, really, just something.)

This is a screw-up on the writer's part. She clearly forgot about this by the time she got to the later books. (And, believe me, a lot of readers remembered it quite well.)

In other cases, things aren't really screw-ups, they're because the reader invests a lot in a character and thinks the character is "just like them." Then the character does something that "they wouldn't do", and the outrage begins.

But real mistakes do happen.