Bad Luck

 Don't walk under ladders.  Seriously, what kind of an idiot ducks underneath a ladder while someone is up there trying to work?  Go around, you idiot.

Don't break a mirror.  Those things can be expensive to replace, and nobody wants to deal with broken glass all over the floor.

Don't step on cracks.  You might trip over uneven sections of the pavement, and it's much easier to avoid them altogether than it is to stop and look closely at each one.

Don't spill salt.  If you do, you'll have to clean up a mess of spilled salt.

Don't hand someone scissors.  Those things are sharp, and you might accidentally stab someone in the hand.

Don't put your shoes on the table.  Shoes are dirty.  What idiot comes in from outside, takes his shoes off, and then puts them on top of the table?

Don't mention Macbeth.  Dropping Shakespeare into conversations just makes you look pretentious.

Don't let a black cat cross your path.  Pick him up and snuggle him instead.

Don't seat thirteen people at a dinner table.  That's way too many people to have to interact with.  Who even has a table that big, anyway?

Don't light three cigars with one match.  By the time you get to the third one, the match will have burned down to your fingers.

Don't leave a pin on the floor.  Pick it up.  You don't want to step on it later and get it stuck in your foot.

Don't let the horseshoe above your door hang down.  How are you going to open the door when it's stuck against the horseshoe?

Don't let one bird fly into the house.  Birds shit on stuff.

Don't open an umbrella indoors.  Those things open quickly, and can knock things over.  What's it going to do, rain in the living room?

Don't walk out a different door than you came in.  That's not where you parked your car, dummy.