So I've been trying to track down the source of some really nasty distortions in the bokeh of my 70-200 lens. You can see the distortions on the left side; they appear as diagonal lines running from top right to bottom left, and pretty much ruin an otherwise great bokeh.
Of course, It never occurred to me to suspect the UV filter that I put on it when I bought it. But when I checked it out at the suggestion of the folks in the 70-200 Flickr group, I discovered that's exactly what it was.
No more cheap filters for me.