Just to add more information about lithium-polymer batteries, they can also puff during high discharge rates. For instance, capturing video or sustained bursts.
This is well known amongst the electric radio-controlled model airplane community where the discharges can be very high. Those batteries are frequently labelled with a "C" rating, which gives you the proper amp rate for charging and discharging.
Unfortunately, the camera battery industry doesn't list this information, which can also be an indicator of the quality of LiPo batteries. My guess is that third party batteries are less expensive because they have lower C ratings.
All of that is why I never charge in the camera, and why I only buy OEM batteries...