I've decided to compile a page of improper installations and bad advice. Do not follow the following examples as a quick way to cheat safety.

Here is some text I've borrowed from another site.
Just for a minute consider what doing that would do even in the best of circumstances. Let's say that even with the lights off and you struggling to get the generator running and hooked up that you actually remember to throw the main breaker off, isolating the box from the main power line. As soon as you stick the generator cord into the wall outlet you will have powered the entire panel box with 120 volts through the 15 or 20 amp breaker that serves the circuit to which the outlet is wired. Everything in the house that was on when the lights went off will now try to pull power from the generator through that cord and outlet, through the breaker and distributed throughout the panel. It's likely the breaker on the generator will keep tripping until you've trimmed the load down considerably. Anything 220 volts wont work.
But what's even worse just think for a minute what could happen if, with the power out and the room dark, that you have a small lapse of attention and leave the main breaker in the on position while you power the house this way. The power from the generator will travel out the main power line and should a worker for the power company be working on that line somewhere in the neighborhood you can electrocute him. His world will be over and yours will collapse. It just plain isn't worth the risk. Special switching gear, called a transfer switch, must be used from generator to house and must be installed by a licensed electrician. Please do that so not only you but others will be safe. The electrical codes were developed the hard way. Following them is a minimum standard. Don’t get caught on the wrong side. Electricity can turn on you at the speed of light.

I've visited people who have no respect for electricity, no knowledge of the danger of what they are doing, and have taken short cuts when connecting a generator to power their home. In some cases the work was even done by a licensed electrician, with simple instructions never to turn on both breakers at the same time. Of course the standard line, "If anyone asks, I had nothing to do with this" was always present. Here is an excellent example of what not to do. Forget to turn off a breaker and you can imagine the possibilities. I've emailed the person and he claims to have consulted with a licensed electrical contractor and was instructed on the wiring and installation of this solution! He also informed that the neutral wire was not needed as he was not powering any 240VAC appliances. This is exactly opposite to what is needed. Neutral is essential for all 120VAC appliances. He should have taken his own advice and hired someone competant (clearly not the same electrician). Why risk damage or injury when it should be done properly? Nice generator, but things rapidly go wrong. Installing a transfer panel and using a proper cord will eliminate most of these problems.

• No interlock between 30A breaker and main panel breaker. Possibility exists for backfeeding generator to main power grid, potentially electrocuting line workers, damaging the generator, or starting a fire.
• Exposed AC on end of cord if breaker turned on without generator connected. Potential electric shock hazard. This goes for use of any double male extension cord.
• Neutral should be connected in main panel. It carries return current for the two live lines (red and black).
• Ground used as neutral. The ground line is for safety only. Under normal use it should never have any current flow.
• The generator should be modified for this use. Neutral and ground lines should be isolated. This likely is a simple wiring change at the electrical outlet panel. The ground line ensures the chassis and frame of the generator remains at ground potential to eliminate possibility of electric shock. If it were a stand alone generator at a work or camp site, ground and neutral lines should be connected in the generator, and its chassis connected to a proper ground rod.

Here is another dangerous installation. The home owner emailed asking if this was a safe manner in which to operate the generator. Another example of an irresponsible electrician giving bad advice.

One more example of someone who should have his internet access taken away. Some people either have no clue what they are doing or have absolutely no respect for safety. Imagine what would happen if someone forgot to remove the "dryer shorting plug" before restoring main power!

When will people learn? Here are even more examples of what not to do. Don't get fleeced by fancy pictures or big words. Despite pushing safety, these people are giving instructions on how to improperly connect to a home electrical system. There are warnings that doing things improperly could result in death, manslaughter charges, and so on! While the techniques will work, the underlying dangers remain. The risk of "What if?" is very high. Anyone with half a pea sized brain would not recommend this type of installation to others! Bad installations 4 and 5. Some of these pages are no longer available, but I'm sure there are many other irresponsible and dangerous examples online.

I received an email from someone who said his brother hooked up a floating neutral generator through a double male cord into a 3 wire welder outlet. There was no neutral connection supplied. He destroyed a microwave oven and other items before starting a fire. The email actually asked how he could safely connect through the welder outlet! My response was that it couldn't be done. It would have cost less money up front to purchase a proper panel than replace damaged items due to improper connection. He still wanted to know how to back feed the outlet regardless of the blatant safety problem and suggested ways to make it work. Some people just don't get it and will never understand. People's lives must have very little value where ever he lives.
In another email an owner described how he connected only two live lines and ground to his main panel. The neutral in the generator was floating. Measured voltages in the house were as high as 180V on one live line. This resulted in considerable damage to equipment and even a small fire.
Always ensure the neutral wire in the generator is connected through the 4 conductor extension cord.