First off, I typically fly commercially once a month or two for business and vacation. Almost all of that is with Southwest, which has first-come-first-serve seating.
Depending which one, it could be one of mainly two things:
1. If I end up sitting behind the wing, I shoot through the jet exhaust, which is a fairly collimated stream and produces a sharp blur.
2. Shooting directly down, I go through either 2 or 3 panes of plexiglass which causes a sharp blur as well.
Almost all of the time, it's a pain in the ass. I typically get stuck toward the back because I'm lazy and never print my ticket in time. Sometimes, it works out just right - a rare example is the photo posted in the link:
Here you can see I'm clipping the bottom of the window at the bottom of the frame and the top of the jet exhaust in the top of the frame.