Mistake #1 aiming your panels wrong
Lets say you get the notion to do your solar panels your self just decide you dont care about the correct way and dont want to follow any of the possible thousands of people out there with advice or knowledge and you face your panels North. Oh my. Big mistake for best bang for the buck. The only time you will want to face them that direction is if you live in a dictator neighborhood that wont allow them in front or something goofy such as this . And then i would give you the advice to just move to put them out of power but that is another story. If you can manage it would be much much better to face them the way that the sun is most of the day.
Mistake #2 Not finding a stud for your panels to attach
This when you are screwing the rails to the top may feel secure once you get them up . But this significantly lowers the strength your panels have to stay on that roof . Imagine a force of nature such as a hurricane or tornado. Your panels are a whole lot more likely to be ripped off the roof than your neighbors who spent a few extra minutes to find the rafters .
Mistake # 3 incorrect wireing
You faced your panels the correct direction found the proper supports in the roof . But you decide to be cheap. You get undersized wire and you have created a bottle neck and this will lower the efficiency of your panels (possibly they may not work) Electricity is like a highway . A 1 way road and your the unlucky one who gets behind someone doing 45 in a 65 this creates anger and makes you heat up. This is the same way that the electricity works. If it doesn't have enough room to flow it bottlenecks creating heat . Heat is wasted energy and if sized to small a fire hazard.
This is a few mistakes you can make when designing a solar system . They are a lot more smaller mistakes but these mistakes are some that have big impacts to the panels life or efficiency. When designing your own solar setup. This is something to think about .