Artifacts with flickr photos
I've uploaded a picture to Flickr demonstrating the artifacts you get from Flickr when submitting a photo that requires resizing to fit their maximum horizontal resolution of 1024 pixels.
I think they're definitely applying a second-stage sharpen process, rather than the sharpening being inherent in the choice of spacial filter used. This discussion came about because of a discussion on Haddock on the ways in which Flickr can mess with your originals.
It also seems that Flickr will strip some of the image metadata in the JFIF headers that relate to the way in which the image should be displayed (such as white point), thus causing differences in rendering between an image which has been uploaded and downloaded via Flickr, and the original, untouched picture.