Anything that can go wrong, will go wrong. And I'm not just talking about the bad things you do to your characters. Welcome to writing! Murphy is a little pain in the rear and he will strike when you least expect it! So make sure you learn his laws as they apply to writing.
Murphy's Laws of Writing:1.) You come up with a really cool idea... YAY shiny... only to discover it's been done a million times. Not so shiny anymore is it?
2.) After making a command decision to take your story in a certain direction, you commit the words to your draft. Only then does a better scenario present itself.
3.) You write in a side character only to later discover you don't need him/her.
4.) Halfway through a draft you'll learn something really valuable about your characters and/or plot that would have helped you from page one.
5.) You give advice to a critique partner and days/weeks later discover that you should listen to yourself more often.
6.) That line you thought was really cool... yeah now you're cutting it. Either that or you're making it completely unrecognizable in edits.
7.) You'll write something absolutely brilliant. The next day you'll wonder what the hell you were thinking.
8.) Find/replace is your best friend... until you realize it's become your worst nightmare.
9.) Corners aren't just for bad children. They're for plots too!
10.) Writer's block is not a myth. It's a reality. And it will strike at the most inopportune time. Especially when you have a deadline.
BONUS: After you send a query/manuscript to an agent/editor you'll find a typo, several if it's a manuscript.
Have any others? Feel free to add them in the comments.