I hate it when writers slip a dream sequence into a novel. I know they are trying to show the characters inner fears, desires, etc, but it is boring, and my eyes skip those paragraphs so fast that I almost reach the last page. I actually grow bored listening to friends describe their dreams - sorry, friends, I do try not to let my eyes glaze over.
So it is probably one rule that, according to me, should never be broken.
But this is subjective - other readers may like this skip into fantasy.
Another rule, according to me, is to cut out the word 'was'. It is simply dull, lazy and easily corrected. Do a 'find' on Word to discover 'was' and come up with another verb.
But I'm only an editor, so what do I know. The Guardian has asked 14 writers to provide rules to writing. They make interesting reading with advice from take care of your back to never modifying 'said'.