A water leak caused some problems with my
iPhone 4S. The camera no longer worked, the wifi antenna didn't work, and the LED light from the camera was on all the time!
IFixit's tools and repair guides helped me disassemble my phone and clean out the insides. The light is no longer on all the time and I know how to replace the camera if I decide to do that.
The magnetic screw mat would come in very useful for this repair. I lost one screw in the process and I was lucky it wasn't a vital one!