My MacBook Pro battery (model: A1281) gained extra weight today due to unknown circumstances.
I always keep my laptop running on power source without removing the battery even when it's fully charged. I don't shut down the system, I just close the screen and leave it on sleep mode. Yesterday though I left it downloading Apple Software Updates and went to sleep. When I woke up today I found the cooling system working pretty hard and the whole laptop unibody was quite hot but what made me certain there's a problem is the trackpad. Those who are familiar with Apple's buttonless trackpad know how it feels when you click anywhere on it, it simply clicks, but mine did not click no matter where I press. At first I thought there's a problem with the trackpad itself but then I saw the bottom of the laptop to see the battery cover slightly curved. I opened the latch and the battery popped out of its place. Turns out it got deformed and swollen in the middle pressing against the trackpad from the bottom.
Curved battery cover
Battery cover with latch closed
Battery cover should align horizontally over the battery
I have no idea what caused this problem. I'm just thankful I noticed this problem before it got worse.
Even with this defect the battery's running good with only 120 cycles and over two and a half hours lifespan.