So imagine a movie that opens with a child being kidnapped. That kidnapping sets the plot in motion, and the search for the child is the main action of the film, with the identity of the kidnappers being a central mystery. Now imagine, two minutes later, a scene in which the mother, before the police come to tell her about her missing child, is talking on the phone in her living room, and in a mirror on the wall, the child is plainly visible getting a drink of water in the kitchen.
If you saw that, wouldn't you spend the rest of the film wondering what the mother's connection to the kidnapping was, and how the child was back at home after we saw them being abducted? Wouldn't it be a little infuriating if the film never addressed it?
Of course, from the filmmakers' point of view, none of this screwing with the audience was ever intended - during the editing process, they moved the kidnapping up to the start of the film, but they needed the content of the phone call to set something up in a later scene. Feeling that a flashback might be confusing, they just edited all of the shots of the child from the phone call scene, and then pretended that the phone call scene was taking place after the kidnapping, but before the mother finds out about it. Unfortunately, they missed the reflection in a couple of the shots, and they wound up leaving a deeply confusing image in the film.
Can an observant viewer be blamed for letting the awareness of that reflection colour their impressions and expectations of the rest of the film? This isn't like seeing wires attached to monster heads, or a boom mike drifting into the top of frame. This is an error that doesn't look like a mistake, and has the potential to drastically change the meaning of the film - possibly the most severe kind of mistake.
Which brings me to Curse of Chucky.