People are truly creative when it comes to home repairs. We will find ways to fix a leaky faucet or some duct work in innovative ways with household items, or we develop creative wiring methods to provide electricity where we need it. Truly concerning is our ability to repair items in such a way that can cause serious harm. For this reason, I would like to mention some things that I see all to frequently, so you can avoid these mistakes.
Note: There is not enough room here to go into the details of how to accomplish these tasks. Besides there are several good books, which have easy to follow steps and diagrams. I have found repair manuals like these in used book stores, but they are not too bad at other places. These volumes are great for homeowners who want to tackle projects themselves.
Extension cord outlets seem to be a favorite means of delivering power to equipment outside. I have seen small extension cords cut in half and passed through the wall where there is a small opening. The cords are then spliced back together with tape, so water can get in and short out the circuit. Most cords of this size are not rated to handle the type of usage you would be planning to handle, so they start to overheat, and the casing will melt away. If you need an outlet for Christmas lights, I recommend installing an outlet in your soffit. The soffit is the board under the overhang of the roof. Find a junction box in the attic from which you can run a wire. Look for where a rafter comes down to attach the outlet box to it. The outlet does not need to be GFCI in this location, but it would be a good idea to go ahead with a GFCI outlet here. Attach wires to rafters and joists with wire clips. If you need an outlet for tools outside, the easiest method would be running the wire through a conduit tube to the outlet box. Here you would definitely need a GFCI outlet with a cover. Make sure that the conduit and the box are securely attached to the wall. Any wiring running outside the building needs to be secured with clips every few feet. The other major wiring problem I see is using the wrong junction box, particularly with no cover.
Putting globs of silicone seems to be the common solution to a plumbing leak. The really amazing thing is that most plumbing parts which need to be replaced for a leak are not really that expensive, but here is the catch, you need to buy the correct part. If you know the manufacturer of your fixture, finding a replacement part is much easier, but I have found that going to a good hardware store you will find some one to help you with finding the right part. Gaskets, seals, and o-rings eventually fail with age. However, sometimes pipes or tubes are knocked about causing damage to their threads or to their walls. In this case you will want to buy a new one. Some mysterious leaks in the bathroom are from the wax seal under the toilet. Trying to make some part that looks similar fit and do the job will cause more issues.
About silicone, one type of silicone is not always acceptable for every use. Look at the labeling to ensure you are getting the product that will work. The best product might not be a silicone. There are latex and polyurethane caulks which are far superior to silicone for an application. Latex caulks are great for paint jobs, and polyurethane caulks are great for outdoor situations.
Duct tape is not for ducts. If you need to repair a duct in the attic, use the shiny tape which is meant for this job. Also a duct is not a duct, meaning that there are actually several different types of duct, so you need to make sure that you are installing the right one for the application. Ducts for different uses are not designed to attach each other.
The last common mistake involves fasteners. Screws, nails, and bolts come in different sizes and types. There are special nails for attaching roof shingles, and there are specific screws for drywall. We like to use what we have on hand (or what we can easily find in the garage or shed), than what is the best fastener for the job. I have seen people nail hinges into the cabinet. What made it worse was the fact that the nails were too long, so you could jab yourself with them inside the cabinet. Some hardware stores sell boxes with a variety of fasteners. This is a great deal if you do not want to buy many different packages. The proper fastener will make the job easier to do (and undo).
Maybe I should say that all of these mistakes come down to planning, preparation, and execution errors. Think about what needs to happen, before you start. Then think about how to correct it if you do need to redo it. (How will they get that nail out of the hinge?) Plan out all of the steps, and you will be fine.