Heres a few good articles about how bokeh works, look especially at the two lense comparisons 2/3 rds down in the first one, and the mirror lense example in the second one. Getting onion/doughnut bokeh, while annoying in cases, is quite common, if not always pronounced enough to notice.http://www.luminous-landscape.com/essays/bokeh.shtmlhttp://www.bobatkins.com/photography/te ... bokeh.html
There are all sorts of weird effects that occur when light bounces back and forth between semi-reflective surfaces. Perhaps a pattern somewhere in the image (though not necessarily in focus) interacted with the optics to create the effect that only shows up in the very out of focus areas.
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Just wanted to chime in with two images that shows what actually can happen if the circumstances are correct. Even though its not interference by adjoining optical surfaces, its a rather extreme example of flares due to internal reflection inside the lense.
The first issue of the Canon 24-105/4L IS USM which was later taken back and exchanged... IIRC the cause was lacking paint or internal flocking on the internal barrel.
A 300D with a kit-lense pointed at the sun, could be some refraction in the microlenses or something due to the angle of light from the lense, but thats just speculation, because I really cant imagene a lense that would go through Q/A showing that level of refraction.