I'm gonna drag this out a bit for safety's sake. After studying a bit I found that Devrees who is an established member had this to say about discharged cells.
Absolutely NEVER Exceed 4.3v/cell, never Discharge below 2.7v/cell, never puncture the cells.
With other cells, they often give warnings similar to that, and you can break the rules as much as you want, and suffer nothing, or maybe a little drop in cell capacity. LiPo is NOT like other batteries in this regard, when you break the golden rule, often even just one time, you are effectively telling the battery to fail into a ball of fire.
If you wish to use LiPo, you MUST follow that rule. No exceptions here. No room for error. No Oops, my charger reset, no excuses.
The most simple way to follow that rule is to use the wide range of awesome microprocessor controlled chargers made for LiPo batteries. They will never let the first part of the rule be broken.
For the second part of the rule, you need to watch your pack voltage while riding, or setup a LVC if you are the forgetful type. If you do ever deep discharge a cell, you are in no immediate danger. As long as the cell doesn't reverse polarity it wont explode just from excessive discharge. However, when you go to re-charge that cell, this is when you will see the fireworks. If you want to attempt to re-charge a deep discharged cell, do it in an area that you wouldn't mind making a bonfire. Maybe invite buddies over to watch. You can always stab it to make it go off if it doesn't go by itself. A deep discharged cell should never be trusted again in a string with good cells.viewtopic.php?f=14&t=22949
Read from the link above for more data.