Hoya filters are noted for having such problems. Personally, I've seen three such problems with Hoya filters due to the retaining ring not being properly seated or coming lose. Granted, it's not a common occurrence, but it happens.
Shel > [Original Message] > From: Frantisek > Even more, his filter might have been badly seated. I wouldn't be > surprised at bad results when the filter is not planparallel to the > lens/film. And a lot of filters can get loose under use/abuse.
