This happened the other night when I d/l'ed and ran the Safari WebKit Nightly Build, and it's also happened once or twice before (under circumstances that I can't recall specifically, but that dealt with Safari installations)...
When I installed the Nightly Build, something caused my AdBlock For Safari database in /Users/Your short name/Library/Safari/LocalStorage to become corrupt.
If you, like I, have spent considerable time and effort configuring your AdBlock database so Websites you regularly visit render in a fashion that pleases to you I most emphatically recommend that you keep a current backup of /Users/Your short name/Library/Safari/LocalStorage against, so to speak, the fall of night.
The first time I encountered this issue it took me hours to recreate the browsing experience I had created for myself and lost.
I filed a bug report with Apple many months ago, but nothing seems to have come of it. Additionally, AdBlock's developer has disclaimed knowledge/responsibility.
