I've got to share a funny little story that just happened to me. I recently got a YN-568 and a pair of YN-622 wireless triggers and talked my daughter into being my test subject this afternoon. We were shooting on my porch with a forest of green trees as the background, a partly overcast sky providing light to my daughter's right, and my speedlight bouncing off a white ceiling to my daughter's left and slightly in front of her (and just around a corner from me).
The first shots looked very encouraging, then for some reason the flash output seemed to drop off drastically. I couldn't figure out what was going on, so I just turned up the flash and shot a few more. Then all of a sudden the flash was overpowering - what the heck
Turned the flash down some and the next few shots were good, then the flash power dropped way off again.
When I walked around the corner to look at the flash it was obvious what was going on - I have ceiling fans on my porch and I had sat the light stand beneath one, the slight breeze was slowly spinning the ceiling fan and it was covering up the speedlight as it rotated. 




