Resources that didn't load (ie: "Not found" in Safari's activity window) should be more easily distinguishable in the inspector besides being lumped into the "other" category. In the network pane perhaps the text could be red and display no pill and when selected in the lefthand source list, instead of showing a blank preview pane, we could mention that it failed to load. Even better might be to have a "Failed to load" heading under the source list and group them there.
<rdar://problem/5712845>
Resource loads that failed show up as red treeview entries.