Exploding bokeh -- NEX-5N & Vivitar Series-1 28mm f1.9 OM. Click for larger.
It looks almost like the zooming effect you get when you take a picture while zooming the lens. Even though I have had three copies of this lens before (still have two), I have never noticed it before. Just thought it's kind of unique. This only happens at f1.9 though. By f2.8, the effect is mostly gone.