So, lets be honest. A lot of home owners do their own wiring. To be fair, a home electrical system isn't too complicated when you know what you're doing. The problem is that there are actually many different ways to wire a house that will WORK but aren't necessarily SAFE. I've seen a lot of situations where the home gamer gets the lights to flick on and off and calls it good only to find out that their home is wildly unsafe or even a fire hazard.
So I'd like to address the second most common electrical mistake I find outside of the electrical panel itself and that's reversed polarity at an outlet. This is one of those things that you would never know is wrong if you're just plugging in the toaster but electrical systems are designed to make them as safe as possible and reversing the polarity on an outlet is one of those common mistakes that just needs to get taken care of. Luckily its an easy fix but lets talk a little about what this actually is and why you don't want it lurking in your home.
First off, I am not a licensed electrician so, to appease the lawyer gods, don't implement any of the information here without first getting a licensed electrician's say so and if you're in an area where you can't do your own electrical work, don't do that either. As a matter of fact, if you're in an area where regulation has rendered your ability to take care of your own house and home moot, I'd recommend not going out into public unless fully padded. You get the point.
Now, there's a lot of electrician jargon and shorthand that makes their work faster. Most people don't know any of that so I'll do my best to explain things in layman terms. If you're an electrical engineer and want to get into the nitty gritty about grounded vs. grounding, neutral balancing, split phases and the like; this isn't for you. Here we're talking about Homer Simpson level mistakes.
Your home runs on AC or Alternating Current which flows in both directions through the circuit but for our purposes here, we should think of electricity flowing in one direction from the un-grounded or "Hot" conductor (commonly black or red wire) to the grounded or "Neutral" conductor (commonly white). There is another wire that should only ever be used as a safety and that is the bare copper ground.
Every switch or outlet in your home is like a gate that opens when you connect the contacts or plug in that hair dryer. This allows electricity to flow from the un-grounded black Hot wire to the Neutral. When we "ground" our circuit what we are really saying is that we created a path for electricity to complete its circuit and return to its source. Electricity in a circuit will always try to return to its source and there are three ways to do that from the Hot side. Through the neutral wire (the way its supposed to), through the grounding wire (the way it can go if electricity escapes the Hot side but can't return through the neutral), or though you if you're touching the Hot wire and you happen to be "grounded". That means you're in between where the electricity is coming from and where it want's to go. Back to its source. Because this can happen through the actual earthen ground is why that whole ground thing got its name. But I digress.
So, reversed polarity is when the side of an outlet that is supposed to send electricity to the appliance is connected to the the returning wire and vice versa for the other side. As mentioned, your home runs on AC current so things will still work but for our purposes the business end of the circuit, the Hot side, will be feeding whatever you plug into the outlet from the Neutral side and this can lead to safety problems.
Say you're making an English muffin for breakfast and the dang thing gets stuck. You flip the switch off and stick a fork in it but the internal toaster switch cuts power from what it thinks is the Hot side of the circuit but now power is coming in from the neutral side so the toaster is still energized. You've forgotten your bunny slippers so you're bare feet make great conductors and you get a full 20 Amps of Zeus's solution to thing's Zeus doesn't like. Breakfast ruined.
Now, having survived your hot breakfast, you go to change that light bulb that has been bothering you. You ascend the ladder with some pride seeing as you're still a little wobbly from your previous shock but your electron induced state causes you to touch the threads of the light bulb socket. Normally, this is Neutral but today, its wired Hot and Tesla's angry pixies go charging through you and that aluminum ladder to get that circuit completed.
I'm having a little fun with the scenario but, in all reality, its a pretty serious matter. This is a simple mistake that can be dangerous and often years of use with no incidents lead to a false sense that everything's OK. Often when a home turns over to new owners, its because someone has young kids so I inspect every home with the worst case in mind. I don't mess around with safety but, I pay particular attention to things that could hurt toddlers, young children and elderly people.
So what can you do? Go to any Home Depot or Lowes and pick up an outlet tester. You can get a pretty good one for around 10 or 15 bucks that will be 99% effective. These aren't foolproof and there are ways to trick them but aside from opening up the outlet, they do a pretty good job. If you do find reversed polarity outlets get an electrician involved. Its a simple fix that doesn't require any additional parts and the labor shouldn't be much more than a trip charge. If they're taking more than a couple minutes per outlet, they're slow rolling it. All that is required is to swap the Hot and Neutral wires to the receptacle.