Oh, man, I hate those! Once, I was fighting a LookupFailed when a 404-NotFound swooped overhead. I tried to run away, but I tripped on a Runtime error and crashed right into a nearby desktop.

That's pretty good...sigged.
As far as Lookup Failed, I've had those in the past, but fortunately haven't run into them recently. I did completely re-install Skyrim. After that I carefully added mods bit by bit until I got most of my previous load order back together. Never did find out the cause of it though.