'non-existant action' in dblog

I'm getting sporadic entries of 'non-existant action' in dblog for perfectly good actions.

Location: http://mysite.com/drupal/system/files/imagecache/featured_block_4/produc...

And yet, when I go to that URL, I see a perfectly resized image. So the errors seem to be false positives. Any idea how to get rid of them? I get one or two on almost every page load.