Simple part, big problem (replacing the home button on iPhone 4)
iPhone can survive falling into a huge pile of snow, but falling into a melting spring snow was like a cold bath for the phone. Everything was ok, but the home button wasn't responding which made iPhone practically useless until I turned on an assistive touch feature which can emulate the home button. The most annoying part began after a week or so when the button started to respond chaotically which caused voice control to activate. Three weeks while the button was on the way were like helll.
Apple used proprietary pentalobe screws to keep geeks away from the interior and it make some sense. iPhone is quite a complicated device, without a repair manual it's easy to make a lot of mistakes. The button probably is the simplest piece of the phone, but it takes about 40 steps to get to it and 40 steps to put everything back. Fortunately everything went well.
First of all, I would not recommend such difficult repairs to beginners, as I said, it's quite easy to make things worse. Also if you have a warranty or an apple care plan and the problem wasn't caused by you it's better to go to your authorized service center (or to an Apple store for all of you who are happy to have any in your location). Otherwise if you have straight hands and all the necessary tools and parts you're good to go.
The most important part for all repairs is not to try to cut corners, disobeying the manual might cause great problems. As for the iPhone 4 it's quite easy to break the cables connecting the front cover to the logic board. The screen and digitizer cables should be put through a hole during the reassembly otherwise they might get too short to reach a logic board. Some actions to remove glued parts require some amount of force, which should be applied with great caution. And for cleaning the inside I'd recommend much widely available alcohol (spirit) rather than expensive liquids with funny names. And don't try to open pentalobe screws with flat screwdrivers or knives, better get a normal tool.