I removed a battery pack from a mid-2011 MBA. Two of the lithium polymer cells had expanded so much that the upper case was bulging out about 0.5cm. The bulge un-bulged itself as soon as I loosened the screws on the lower case. There doesn't seem to be any significant damage to the hardware. Pretty wild. Also, despite this repair being listed as "Difficulty: Moderate", it's about the easiest thing you can do that involves opening the bottom cover.