I'm assuming you're not talking about one that already looks like a football -as long as the lipo doesn't start swelling more while you're charging, it may be OK. If it starts to swell while you're charging, stop immediately and take the lipo somewhere where it can't damage anything if it does continue to swell and go off.

If you're using a good lipo capable charger in lipo mode for charging, you shouldn't be able to overcharge it, voltage should not get too much more than 4.2v/cell.

Capacity is another issue. Some lipos are under rated, it does happen.