Made a Adafruit Flora unusable
42 pixels in the strand all set to bright white.


This exceeds the power supply capability of either a USB port or the Arduino's regulator.

Likely the circuit starts out working, then once the LEDs are all on drawing high power either a protective device such as a polyfuse / PTC fuse activates trips or the supply simply sags under load.  At this point the circuit either shuts down or starts misoperating, the ATmega may go into brownout protection mode, etc.

Disconnect power to your LEDs and you will probably find that the Flora is fine.  

You will need to use a suitable higher current power supply if you want to light more than a few pixels of this type at any given time.