I put my 3GS on charge and this happened!! Pulled out the charger as soon as I noticed what was happening. I think the ballooned battery was close to exploding. I've never tampered with the device and have always used the official Apple charger. Granted it's an old phone, but this should never happen. Ever. Was a bit scary as I have young children playing around the area I put the phone on charge in...will definitely be more cautious about leaving my 6S on charge unattended.