I’m probably displaying my photographic ignorance here but I just figured out one of those mildly annoying technical issues we all live with in various forms.
I have a Tokina 150-500mm f/5.6 AT-X (http://www.kenrockwell.com/tokina/150-500mm.htm – just for you Orionid) manual focus lens in Nikon mount with an adapter for my 7D. I’ve had it for a few years and some times I get “stringy” or “double line” bokeh from it. Annoying but it is a nice lens otherwise, I just lived with it.
Which explains the physics of it, and a short google later I found that it can be caused by UV filters. The universal protector has become a liability in this case, go figure. I took the filter off (all 95mm of it) and voila, problem solved.
You can see the effect in my entry for “watercraft” titled “Get on. Ride hard.”
It is still a MF lens and difficult to use for action (like that jet skier) but I have always liked it and it just got a little bit better.