T3i - BODY PANEL ATTACHMENT QUESTION
I have not taken apart anything newer than a G9 or 300D, but in my observation, Canon does not use screws to pull body pannels together the way laptop makers do. Instead, they snap in to place and then the screws hold the locking tabs into the frame so that the screws are generally bearing minimal loads and won't work loose over time (an issue with a few screws on the G9, actually, because the designers violated this rule).
Instead, it looks like your camera was assembled incorrectly and the screw was driven (hard) into the metal or plastic frame within the camera. Return it for a new one if possible or get it repaired under warranty.