it happened to me once. mine was a 4s lipo. one reading at 0.8v the rest around 3.8. so what i did was to charge the bad cell (0.8) through the balance port. just stick a wire in there and charge it with nimh setting and very low amp until the voltage reach around 3.8v then cycle the battery like normal. seems to work fine.

in your case you may want to discharge the 4.2v cell first just to be safe.