Not sure what would cause this... it could well be an attack. You could try and make a construct an html page with the png to see what it looks like (visually) or try to see if you can unbase64 encode it using a utility to see what the contents are (without running it on a browser).
I've seen similar 404 errors after a visitor arrives as the result of a normal Google search.
The wanted page and its associated images are downloaded normally.
"GET /url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAADgAAAAOCAYAAAB6pd..... 404
The very long base64 code starting iVBORw0... can be decoded to this rectangular image:
The icon to the right hand end is a Google icon, so the phenomena may have something to do with Google.
Best regards, Eric.
Author and all-around good cowpoke
Joined: Mar 22, 2000
AHA! That has got to be it - maybe Google made some sort of change around October 23 which generates this spurious GET or some other site is leaching off of Google and screws up the link creation.
Now who can I talk to at Google....
subject: Mystery GET with url(data:image/png;base64,iVBOR