I replaced the battery in my iPhone 4 because it seemed like a moderately simple repair for me to complete. The old battery no longer held a charge, so I decided to try to give the device a second life.
My repair almost went entirely to plan. It turns out that the original Apple battery that the device shipped with had expanded, and it was pushing its way out of the battery compartment in the iPhone. Because of this, the plastic tab under the battery tore while I was pulling it, leading me to use a random tool that was laying on my desk at the time to get it out.
My advice is to drain the battery and recharge it at least five times, more than what the iFixit guide calls for. Batteries need to be calibrated in this manner for the "percentage remaining" icon in the OS to be accurate.