Sometimes Safari opens with blank/empty tabs with window titles of the previous session I don’t dare to quit Safari if I want to keep the tabs opened, because this happens quite often on my MacBook, running OSX 10.8.2 and Safari 6.0.2. Is this a known problem?

It is strange because window titles are still there but I cannot reload the page. So I always have to search for the page title to get back to the page.
It's a bug - I've reported it but can't force it to happen. If you find a way to trigger it with great reliability you could report it here or to the source at http://bugreporter.apple.com and it might get squashed sooner when the engineers can see exactly what is failing to cause it.