Swollen battery, cracked screen
The phone came across my desk, and the screen was bulging and busting at the seams (and obviously cracked the screen), so I carefully investigated why. It turned out that the battery was incredibly swollen and distorted, and it had an awful lithium smell. So I carefully disconnected the battery, grabbing some safety glasses and rubber gloves because it looked like that battery was ready to blow—and eventually did when I was trying to remove it it. But once I replaced the battery and the screen, the phone passed every inspection I could throw at it.
After I got the battery out, it was smooth sailing, and I got the phone back to a happy customer.
Always always always, whenever you approach a swelling battery—wear protection. Battery acid is nasty and it burns. Oh, and don't try to use a spudger to pry it out—that's how my battery popped. Luckily, I only punctured the outer layer of the battery and not the inner layer.