I think it's an optical effect, something flare related, shooting into the light like that, possibly with a filter, or even a shiny lens hood, light catching the filter threads, that kind of thing.
Nothing to do with the shutter.
Hello everyone, Ginger//owner of misbehaving 30D here. Thank You for your input so far.
Hoppy, yes, I was shooting on this particular day with a hood, and it is more on the shiny side, but as Paul mentioned, this effect occurs with both of my lenses, hood or no hood, and whether the sun is behind me or not.
I've aimed it at the sky with the light in so many positions around me, that I feel it just can't be a flare. I shoot those purposely with my little Powershot, so even as an amateur I'm fairly certain it isn't that simple. I wish it were!
So that leaves, so far, "crap in the mirror box, on the sensor or even on the focussing screen perhaps"
and shutter failure? It is this same band of light, same halfmoon facing left every time, if that helps diagnose. If it is shutter failure, would that occur over a long period of time? This has been occurring for over a year. Because it only really shows up when I shoot the sky or light subjects, I have been passing//editing//& posting less than perfect photos of everything else and letting this problem go -- that or using other cameras.
That pesky "Error 99", has been occurring very randomly [maybe four times now] over the last couple of months. So far has always been remedied by removing and replacing lens. Once I cleaned the contacts for the flash foot [per instructions on a forum]. Wonder if that is in any way related.
??