I am on my 2nd pair (one for both myself and GF).  Same problem with the first pair, water damage as others are describing.
On the second pair I got smart and analyzed the problem and came up with a simple fix.  Take some vaseline and put a good coat on the rubber seal of the battery bay.  Problem solved!  Stays very dry.  Somehow the water was getting in though the battery door even though its on the bottom and had a rubber seal.  I also lay my brush long ways now vs. vertical.  This way any water doesn't run down.  I have been though at least 10 or more battery changes (I use rechargeable batteries) and at least 3 sets of heads.  I make it at least 3 weeks between charges for two brushings a day and 3 sometimes on weekends.  I owned an original Sonicare and hands down I prefer this model.  I will not even consider the newer fancy ones.  I am going to buy 2 more for future spares.  You can get them for $18.56 shipped right now on amazon with a $5 rebate!  5 stars for a Sweet deal / Awesome  Value! Please I will never brush without a Sonicare ever again, my gums are way more healthly and my teeth are always very clean with very little plague.
Rating: 5 stars