I experienced this problem whilst shooting some products for a company today. I don't know why but here goes...
I just bought a wireless flash trigger (Trigmaster) together with a Nissin Di866 flash. The photo below was shot with the flash bounced w/ shoot thru. I was happily shooting without issue until i set my shutter speed to 1/250. then this happened. i was standing fairly near to the flash when i shot this (flash and shoot thru were on my right). i noticed it also happened on some photos at 1/180. when i moved away from the flash and started shooting other stuff in the house, but away from the flash, it was ok. sometimes, the dark edge appears on the bottom instead when I am shooting landscape. portrait it will appear on the right.
was wondering if any one can help me here. is it a setting, lens, flash issue? photo shot at f4, 1/250, ISO 100 @ 44mm. Thanks for your help!