My band had the same issue, the metal contact on the charger had become corroded, strange bubble shapes had appeared on it, and the charger no longer would sit correct either.
I ended up getting it replaced, the new one works well so far. (Except for the microphone not working) The same effect on the metal is appearing though, and I have been very careful to only clean it using the directions on the Microsoft website and not let any thing else touch it.
The reps at the Microsoft store indicates this is caused by our sweat.