A black cat crossing your path is unlucky - but only because it means that you probably are a cat owner, and as anyone who actually has cats will tell you this sucks. Say goodbye to your furniture because the little freaks will claw the mess out of it. Hope you enjoy having them desecrate your carpets with their poop, which often contains various household objects that you've been missing for quite some time because cats like to chew on stuff. Have fun getting hit up for thousands of dollars each year to make sure the cat, who lives inside your home, does not have some sort of disease that could KILL YOU.
Breaking a mirror will cause you to have seven years of bad luck - but only because that's how long it will take you to get all the little shards of glass out of your carpet, and until you do you'll get stabbed in the foot repeatedly.
Walking underneath a ladder is a certain sign of misfortune - but only because it reveals your own lack of intelligence. Someone as dumb as you is in for a long and painful life full of unpleasant surprises - not unlike the unpleasant surprise you'd receive if a large can of paint plummeted 25 feet and hit you directly on the head.
Seeing the bride in her wedding dress is not a good thing - but only because it's a reminder to you that in just a few minutes, you're going to be committing yourself to spending a lifetime with someone ugly and mean. Do yourself a favor and do what I did - marry someone who's hot, funny and smart - and you can avoid this problem altogether.
You wanna know how I know that people who believe in luck are gullible and stupid? The rabbit's foot. Don't tell me that this myth wasn't started by two smart people pulling a joke on one of their stupid friends...a friend so stupid that he'd actually believe that his life was going to be negatively impacted because he broke a mirror. I can just see it..."Hey Billy! There's a sure-fire way to remove that there bad luck curse off of you. All's you gotta do is...umm...go over there in the cabbage patch...naked...and catch little fluffball. Then, once you catch him, you, umm...cut his foot off! Yeah! You cut his foot off. And then you...uhh...attach it to your key chain! Then you take it with you wherever you go, and as long as you have Fluffball's little foot in your pocket you're safe." Then everywhere they went, the two friends would always ask Billy to go get something out of his car so that he'd have to take his keys out, thus allowing everyone else there to see the little stumpy rabbit's foot, and they'd all laugh. This has college fraternity prank written all over it.
What the ignorant perceive as "bad luck" is in reality the "suck" factor of life rearing it's ugly head. Crappy stuff happens sometimes and there's nothing you can do about it. Deal with it. It's not some cosmic conspiracy to mess up your life because you did something trivially stupid...if it's any consolation to you, life treats the rest of us like crap as well.
See a penny pick it up, and all day long you'll have good luck? What in the heck is a single, stinking penny going to do to change the outlook of my day? I can't buy anything with it, I probably picked up some rare disease just by touching it, and ultimately for every penny I pick up it means someone else lost a penny.