After 3 years my MBP suddenly started growing! Wowsers, now the track pad is popping out! Quick, something is pushing the casing apart from inside - an alien baby is in there - no - the battery is expanding. According to Apple, this can be normal battery end-of-life behaviour! After a brief discussion (computer says NO!) with the helpful Apple genius I decided to try fixing it myself - with some help from ifixit.
With the recommended tools, it couldn't be easier. The new battery was in and MBP was back up and running in 20 minutes.
Glad I didn't leave it any longer. The battery (now out in the greenhouse) is now nearly twice as thick as it used to be! And I'm feeling half as thick as I used to be!
Get the back off the machine as soon as possible if the casing or track pad start bulging. This will help stop any permanent damage to the rest of the laptop. I managed to backup the machine in this state before removing the battery. Don't bother ringing Apple, as soon as they hear it hasn't actually exploded, they dismiss the complaint, unless, of course the machine is still under warranty.