There are a lot of writing rules out there, particularly for fiction writers. Well, to be honest, they probably exist for other kinds of writers, too, but I don't pay attention to those rules as closely. A lot of these rules make sense and they have purpose, but they're also extremely flexible.
I'm currently re-reading Harry Potter, and I'm about two-thirds of the way through Prisoner of Azkaban. (Why does spellcheck not yet recognize "Azkaban" as a word??? Come on, Google, get with the program.) In two-and-two-thirds of a book, I've come up with some of the most breakable rules in fiction, according to JK Rowling.
Breakable Rule #1. Don't use adverbs.
JK Rowling never met an adverb she didn't like, and she uses one to describe almost every piece of dialog. I think she uses these to avoid using cluttered dialog tags. You know, instead of "shouted" she uses "said loudly" and instead of "mocked" she uses "said mockingly." So, potayto potahto, if you ask me.
I counted one sentence with four separate adverbs in it. Part of this has to do with her audience; the early books are intended to be middle grade or early young adult, and kids that age aren't as adept at reading body language. So Snape can't just smile at the kids, he has to smile coldly to get the point across.
HOWEVER: If you're not writing for kids, or if you're relying on adverbs to describe everything, there's a good chance you're using the weakest words possible. To say something fiercely is not as powerful as demanding, commanding, pronouncing, or some other strong-sounding verb.
Breakable Rule #2. If you can't read a sentence aloud in one breath, it's too long.
I've counted no fewer than four sentences with over one hundred words in them. They're heavily punctuated, and they all make sense, but they are looooooooong. Sentences routinely clock in over forty or fifty words, and will change subjects multiple times as they go. But it works within the voice she's constructed.
HOWEVER: If your sentences sound rambly when you read them out loud, they're too long. Cut. Slice. Punctuate.
Breakable Rule #3. Don't use passive voice.
Example of passive voice: "While Hermione was checking that the coast was clear..."
Changing it to active voice: "While Hermione checked that the coast was clear..."
Rowling breaks this rule... oh, I dunno, approximately a thousand times in each book. Seriously, almost every verb that could be active is written passively. Harry never ran, he was running. Ron never waved his wand, he was waving it. Hagrid never hummed, he was humming. On and on and on.
The big reason this one can be broken is because the story is so good. If the story wasn't so perfectly engaging, you'd never be able to overlook this one. There's no "HOWEVER" on this one, because you really shouldn't break it. Rowling gets to. She's the queen.
Breakable Rule #4. Don't break POV. If you're in limited 3rd, stay with that character.
Chapter one of PS/SS is in omniscient third. Chapter two starts in a semi-close-omniscient third in Harry's POV, but it's still not a traditional limited third. Throughout that book, we break POV. The quidditch game when Hermione sets Snape's robes on fire? That's obviously not in Harry's POV. Rowling breaks the fourth wall a handful of times, but then returns to the limited third POV.
Throughout the series she breaks POV, sometimes in a big way. CoS and PoA both start in omniscient third. GoF starts in omniscient, and closes the opening chapter as if it's a dream, sliding awkwardly from one POV to another. HBP has two chapters that Harry knows nothing about, like a double-prologue. The Prime Minister chapter is told in a completely different voice, and then Spinner's End is yet another voice. The POV problems tighten up a lot as the story progresses through the later books, but it's still not 100% consistent.
HOWEVER: If you're head hopping around like mad, it's confusing. If you're going to change POV, make it for a single scene, or a single chapter, at a time. Make the transitions obvious. (No Goblet of Fire omniscient-storytelling-narrator-to-dream sequences, mkay?)
Breakable Rule #5: Show, don't tell.
BLAH. Sometimes telling is the best way to get through some information. Lots of things happening on Halloween (always)? Turn the page, it's several weeks later, and I'll tell you that by saying "Hermione stayed in the hospital wing for several weeks." Boom. Done. This could have been really boring, and stretched out, or we could have described the weather changing as Hermione stared out of the hospital window or something, but just saying it was the best way to get it done. "The end of summer vacation came too quickly." One sentence wrapped up four weeks of the timetable. We didn't need to see it all, we can manage with just being told.
Rowling also breaks this rule repeatedly when Harry passes out. And he does pass out A LOT. After being attacked by Quirrelmort, after meeting the dementors (twice), etc. Dumbledore sits down and just tells Harry everything he missed. (There's another very famous book series that uses the knock-the-main-character-unconscious-while-we-do-the-complicated-stuff-and-fill-her-in-quickly-after trick, but it's not done as seamlessly, so obviously this isn't a one-trick-fits-all kind of a trick) (I use a lot of hyphens. SorryNotSorry).
HOWEVER: If you find yourself glazing over as you re-read your work, it's definitely too boring, and that's a good sign that you're telling too much. Telling is good for glossing over big chunks of time or information that would otherwise be useless to the story. Use it sparingly. But by golly, you should use it sometimes.