I have a friend at work and I noticed that his phone screen was half broken off. I offered to take care of it for him as long as he just covered the cost of the repair parts.
I ordered the parts and they arrived very quickly. The repair manual said that this was a difficult replacement but I didn't have any trouble with it. After I had the new glass panel installed I reassembled the phone. I got all the way to reconnecting the cables and I noticed that cable #2 wasn't sitting as tightly as the original was. After hours of trying to figure it out and my friend getting more and more worried and repeatedly asking if I knew what I was doing I went online to see if anyone else had any problems. Finding nothing I went back to the beginning and did something I should have done in the first place. I checked the model number and found that he had the iPhone 3GS and not the 3G so I had the wrong part. Other then that the repair went great. Would have been a 2 hour job had I ordered the correct part.
As hard as it might be at times you must assume that everyone, friends and family included, is stupid. My friend swore he had the iPhone 3G and not the 3GS and I took his word for it and ended up with the wrong part. He was able to find a local repair shop and do a one for one exchange for the correct part but it would have saved time and stress on both of our parts had I just checked in the first place.