CGR 17360 negative voltage
The battery cells most likely differed in capacity and were discharged to the point where one cell was completely discharged while the other still had some emf left.   This results in the discharged battery being charged in reverse and is called cell reversal.   It can result in a negative voltage on the lower capacity cell.   A quote from wikepedia:


  Generally, pushing current through a discharged cell in this way
  causes undesirable and irreversible chemical reactions to occur,
  resulting in permanent damage to the cell. Cell reversal can occur
  under a number of circumstances, the two most common being:
  
  When a battery or cell is connected to a charging circuit the wrong
  way around. 
  
  When a battery made of several cells connected in series is deeply
  discharged. 
  
  In the latter case, the problem occurs due to the different cells in a
  battery having slightly different capacities.  When one cell reaches
  discharge level ahead of the rest, the remaining cells will force the
  current through the discharged cell.


This stack exchange post discusses the same phenomenon.

The battery is most likely irreversibly damaged.