A knock at the door awoke Sally. She squinted against the light coming through the window and groaned, pulling her warm blanket over her head. But the knock came again, louder and longer this time. With a sigh, she pulled herself out of bed, keeping her blanket wrapped around her shoulders for warmth.
She opened the door again mid-knock, and found Beth standing there with her fist still raised.
“It’s about time,” Beth said with a frown, lowering her hand. “Go get dressed, we’re going for a drive.”
“What?” Sally grumbled. “Why would we do that?”
Beth folded her arms. “It’s about the Grimoire.”
Sally blinked, suddenly alert. “The Grimoire? What happened?”
“We can’t talk about it here,” Jacob said from beside Beth. “Just get ready.”
I’m sure you noticed the problem at the end of that passage—if Jacob was standing next to Beth that whole time, why wasn’t he mentioned when Sally first opened the door? This is another common type of perspective error, when the author fails to share every important thing that the perspective character knows or can sense.
Introduce things to the scene before they're being used
Picture another scene: A burly man in a black suit drags Sally into a parlor, where a sinister-looking old woman is waiting. Clever and threatening words are exchanged, and then the woman tells her burly henchman in the black suit to take Sally out back and dispose of her. The man reaches for Sally, but she grabs the bowling pin on the table and hits him over the head with it. And then the reader goes, “wait, what bowling pin? Since when was that there?”
If there is a bowling pin on the table, that is something that Sally would have noticed when she came into the room. If you didn’t mention it when it would have been visible, then you can’t use it later—that’s the rule. If you get to the middle of the scene and realize that you need a bowling pin for Sally to use, then scroll back and put it in at the beginning of the scene. The important thing is that your character becomes aware of things naturally; the scene won’t feel real, otherwise.
Now, if your character suddenly realizes that there’s a bowling pin on the floor by their feet just a moment before they grab it, that’s okay—there are situations where that’s the best way to go. The important thing is that you show your character becoming aware of it, so that your reader can become aware of it, too:
Sally suddenly noticed a bowling pin, of all things, lying on the ground by her feet. Without taking time to think, she snatched it up and swung it at the burly thug’s face.
This is still a little abrupt, but it is acceptably and realistically abrupt.
Using perspective errors to create tension = bad writing
The examples above are simple mistakes; the writer forgot to mention Jacob and the bowling pin when they came into the scene instead of when they became important. There will be times, however, where you are tempted to break perspective in this way deliberately. Why? Almost always, it will be for the purpose of artificially creating tension.
For example: Sally, after hitting the thug over the head with that convenient bowling pin, runs from the room. She gets out of the building and into the spacious yards around it. But as she runs up to the open gate that leads off the grounds, a guard steps in front of her and points a rifle at her head. She stops and raises her hands. He tells her to turn around and go back. She pleads, explains, cajoles, and maybe even threatens him—she talks and talks and talks, stalling. Then Jacob suddenly hits the guard over the head (probably not with a bowling pin), having crept through the gate while Sally was distracting the guard.
The writer didn’t mention Jacob until he hit the guard over the head so that the readers would wonder how Sally was going to get out of this situation. If they’d known that Sally could see Jacob sneaking through the gate (she was looking that direction, after all), then the tension of the scene wouldn’t have been as high.
I’m not going to mince words—this is a cheap trick, and you should not do it. Your readers will feel cheated, and they’ll be right to; you flat-out lied to them in an attempt to keep them from figuring things out sooner than you wanted. Your characters can lie to one another all you want; if your story is narrated by one of the characters, then you have the possibility of having them lie about the events of the story (though it won’t necessarily feel less cheap if you do this, so be careful); but you, as the author, as the person telling this story to your readers, cannot ever lie. If you do so, it will be bad writing, and that’s the end of it.
Find a better solution
So, when you’re tempted to do this, just don’t. Find a way around it. Rearrange the scene so that the guard is standing near a corner that Sally can’t see around, so that Jacob can appear suddenly without a perspective error. Have the guard come up behind Sally, so that she can’t see Jacob sneaking up. Give Jacob the ability to teleport, for all I care—unless you’re writing creative non-fiction, then you can do whatever you want—the possibilities are limitless! Just don’t lie to your reader.
To make things even more difficult, the same thing goes for the information your character knows. If you’re writing from the perspective of a detective investigating a murder, and at the end of the story you suddenly reveal that he knew all along that their primary suspect was innocent and that his boss was the real murderer, then you’ve cheated. If he knew that the whole time, and we were in his head, then we should have known, too.
Picture Ocean’s 11 written through Danny Ocean’s perspective: it wouldn’t have worked, because the movie ends with a big reveal about what the real plan was. In a movie that's okay, because we’re not actually in Danny’s head—there’s not technically a perspective error. No lies. But in a book from Danny’s perspective, it wouldn’t work. If he knew the plan from the beginning, and we were in his head, then we should have known, too.
You can get around it—take us out of the detective’s head and put us in someone else’s head (as Arthur Conan Doyle did in the Sherlock Holmes stories, relating them through Watson) or have the detective suddenly figure it out at the end instead of having known the whole time. For your version of Danny Ocean, have the plan go wrong and he suddenly has to improvise; or put the story from someone else’s perspective, someone who wouldn’t be in on the plan. Again, there’s tons of options.
Just don’t use the perspective error because it’s easier—you want to be better writers than that.