Oh, you can have a major inexplicable change and have no one in the story remark on it, but you can't blame your readers for concluding that you're an incompetent writer and that the unjustified change was a mistake.
Worse still, your readers might conclude that the unjustified change was a practical joke you were playing on them, as if you were saying, "Oh, were you starting to care about these characters? Were you starting to take this story seriously? Well, here, I'll show you that it's all silly and I can do anything." Of course you can do anything. But your implicit contract with your readers says you won't do just anything-that your story will mean something, even if the meaning is that there is no meaning....
That's very much the feeling I get when watching or reading a story with crappy characterization; especially if it's seemed to be good in that department up till then. That the writer (or writers, if it's a group effort like most television series) has in essence pulled the rug out from under me, and laughed at my trust.
The trick (and I'm saying this as much to myself as to any of you, if not more) is to keep your characters' actions surprising yet somehow inevitable. Your audience should be in a recurrent state of saying "What...yeah, they'd do that," or "Why would they do that...I'd better keep reading (or watching, or whatever) to find out!"
That second one only applies to stories that are going to explain what may seem to be an at first unjustified change, of course.