When I set the DATA_PIN via #define for use with FastLed, I can't compile the code if the pin is set to certain values. E.g. 2 and 3 work, 4 and 5 don't, 6 and 7 do, etc.
Apparently this is some low-level requirement of the pin that needs to be met, but looking at the pinout here I don't see an obvious reason why, and the basic LED tutorials I've read don't seem to mention anything about this.