Using a long (10 second) exposure I found I was getting an odd "reflection" in the picture, where the brightest object leaves a residue in the opposite part of the frame.
You can see it here:
I was still getting it in 4/5 second exposures as well. NB: Using Canon 550d with a 11-16mm F2.8 Tokina lens, at 16mm.
I assume it is some physical thing like an internal mirror or lens thing, but does anyone know for sure?