How did I end up with an overcharged cell?

[aclysma]

Here is my setup:
* Powerlab 8
* FMA balance board (the one with lots of fuses. See picture: http://revolectrix.com/support_docs/item_1401.pdf)

I charged 2 6s 3000 (Hyperion) and 4 6s 3800s (Gens Ace). All have voltage within .15 when placed on the charger.

At the end of the charge I noticed that all charged the charged lipos except for one were in an acceptable state (voltage 4.17 < V < 4.2) except for one lipo. It's cell voltages were something like:

4.25
4.22
4.2
4.16
4.13

In addition, the red strip for the balance leads turned black indicating that too much current was trying to pass through the balance taps.

Any ideas as to how I got into this state? Using this board is more annoying than 6-way split cables but after seeing this I am SO glad I used it!!

Again, I really want to know how this might have happened so that I can prevent it from occurring again.

FYI:
- I immediately put the battery on a discharge until all cells were below 4.2
- I'm balance charging this battery by itself, carefully monitoring it to make sure voltages are not exceeded.