There are a lot of rules for any job and for any creative outlet. In reality, probably none of them are universal, but they are good guidelines.
But there are always times to rebel!
I think if you know WHY a rule exists and you truly understand it, you're in a position to break it. With discretion.
My favourite rule to break is Thou must write in complete sentences.
I like sentence fragments both in dialogue (where it's more acceptable) and out of it. I try not to overdo it, but I love the effect of fragments - especially when my characters are stressed or scared or ticked off. To me it adds to the authenticity.
What's your favourite rule to break?
I'm over at From the Write Angle today talking about Query No Nos - I hope you'll pop over and join the discussion!