See Page H-4 or you user guide.

Warning Indicator:
Incompatible camera warning (non-CLS-compatible)

Cause:
The mode selector is set to or , or the power switch/wireless mode switch for multiple flash units is set to or when used with a non-CLS-compatible camera.

Solution:
Set the power switch/wireless mode switch for multiple fl ash units to and the mode selector to .

Reference page:
F-1

I would also add a loss of power, low power, Speedlight movement (not locked or loose) that causes the communication between the camera and Speedlight to be lost and not properly reestablished.

George
My Nikonian Galleries