Had a charging event this AM.  The pack was a 4 cell Hyperion, 4000 MAH, 30C, fully discharged from the last flight yesterday.  The charger is a Dymond 4 port driven by a computer power supply.  The charger was not powered up.  I plugged the balance tap in first, without the power leads connected.  Plug seemed to insert into the socket normally without forcing it.  It emitted a cloud of white smoke about the size of a peach and quickly got hot.  Pulled the connector apart instinctively and let everything cool.  Checked for dirt or bent pins and found nothing.  Plugged it in a second time and it stayed cool.  Hooked up the power leads and it charged and flew normally, no damage that can be seen.

     So it might have had 1) a speck of some conducting dirt in the connector or 2) The pins were misaligned so the negative common was placed on a positive pin.  The connector socket fully surrounds and guides the plug onto the pins, and this looks impossible to me.

     Anybody have a similar event or a better theory as to what went wrong?

Tom,  
Sounds like you most likely plugged the connector in backwards. They aren't supposed to fit backward but it doesn't take much to fit them that way. If you are using a JST-XH balance board, which is the most common, plugging in backwards is what you most likely did. This is one example of many  http://www.progressiverc.com/safe-parallel-board-for-jst-xh-t-plug.html    If you are using a Hyperion/Polyquest board Then you may have plugged it into a 2s or 3s slot accidentally (2s,3s, and 4s all use the same balance connector but they are wired differently.) That too would cause the issue that you described. Here is an example of the polyquest/hyperion board  http://www.progressiverc.com/parallel-charge-board-for-polyquest-and-t-plug.html. Note the balance connectors look the same for 2,3, and 4s. The common denominator between the 2 types of boards lies at the 4s and higher slots. They are exactly the same on both boards. That's why I always recommend a JST/XH board for the hyperion 4s and higher as you cant make any mistakes plugging them in, unless of course you manage to fit one in backwards.