This is a problem I ran into earlier this year:

http://fahhem.com/blog/2014/01/mint-plus-braava-hacking/

What happens is when it's plugged in and you press two of the buttons together, it gets into a part of the firmware that doesn't like what you're doing and it basically self-destructs. I'm sure it's not on purpose, but what it does is send either invalid commands to the LED driver or overpowers the LEDs and 1 or more of them burn.

Once burnt, they allow too much current through and on powerup they cause the board to brown out and shut off. This causes the short flashing and or faint noise that's heard when you press a button, because that's what it does when it boots up normally: flashes the lights and makes that loud 'happy' noise.

I was able to get mine replace under warranty, but I don't recommend trying it to anyone else. I'm eventually going to hook up a JTAG connector to the board, but I have other things on my plate for now. I recommend any hacking that happens to be after a code dump of the processor. I can help read the firmware if anybody does manage to get it off.