What does it affect?
Seems to be affecting random batches of 3502 / 1142 APs.
The APs boot up with their LED happily flashing away but this merriment is short lived as once they join the controller it will turn itself off. Bit weird. Cue delirious panic from clients who think their APs / power injectors / cables / toasters are broken.
What’s the fix?
Jump into the CLI of the WLC and issue the following command:
(Cisco Controller) > config ap led-state enable all
If any APs did have their LEDs disabled you will see a confirmation list of them turning on. If you don’t see any then you could have another issue on your hands!