Agreed SAXMAN, this week some of mine are coming up completely blank as well.
The root cause was when Safari, (or the CPU in general) was overloaded or interupted and all the tabs were unable to load initially (due to slow connection/one website being offline or sticking, memory limits, OS crashes etc). Once Safari closes, the empty pages get saved in this limbo state and are generally not recoverable after that.
In more than one case, this all started upon waking the system from sleep, whereupon many tabs fail to refresh, even if they still display a title.
Still, if you review your history from yesterday, those pages are generally there for the taking, if you can ID them.
I should add that I have about a billion bookmarks, and the add bookmark process in the new safari is getting painfully slow.
I'd be interested if anyone else can ditto this experience.