Both the above shots were taken with a Canon 5D. The image on the left was taken at 1/200 shutter. The right was taken
with 1/160th shutter. I used cheap wireless sync system that I bought on eBay but have
never noticed this problem before. The shots are for my hotsauce website www.colibrihotsauce.com.

UPDATE

Is it the "cheap wireless sync" system that I bought on eBay?   Maybe, a little. Below is a shot taken with a pc sync cord.
Shutter at 1/200. Not as bad, but definitely still shows the shadow of one of the shutter curtains.