One of the cells in my 8S1P pack I did from two Zippy 4S batteries was spotted swelled. I guess that’s my fault, as I once left board on for the whole night and the voltage dropped pretty low and once board was just lying around fully charged for about a week.
One more maintenance thing that may cause cell degradation is that initially electrolyte is mixed with an inhibitor, that allows battery to be stored for a longer time. This additive decomposes during first 2-3 cycles, which should be done at 3-4C. If the battery is used under nominal load during this period, it may lead to battery damage.
– First of all I’d like to ask about safety. How safe is it to use a swallowed battery? At what degree of volume increase one should consider the battery ‘not usable’?
– Secondly, there are options to go around this problem. I’ve found videos and just suggestions on forums, that putting swallowed battery in the fridge for a day may help. Some people even suggest to carefully puncture battery and let gases out. 
MythBusters demanded.
– Also I now wonder how do Lion batteries go bad? Do they also swallow and might explode or they just die quietly?