Directory listing causes 404 error. Why not 403?

[Дмитрий Сапожников]

Hello. I've begin to receive error reports in Google Webmaster Tool, which said Google got 404 error while trying to list directory with icon images (mysite.com/.../images/). So, I'm checked this directory, "/images/icon.png" opens without problems, "/images/" shows 404 error page. I think, showing 403 will help to prevent errors flood.