I have Acrobat 10. I created a dosument, and signed it, and checked the "Lock document after signing" checkbox. Later, I realized I wanted to make a change to the document properties, so I deleted the document, and recreated it (I have an automatic way to generate the PDF). The new document by the same name didn't have a signature, so obviously there was no way to clear it, but the document behaved as though it was still locked. The only way I found to enable a new signature to be applied was to make a text edit to the document, and then I was able to sign it. Is this a bug?
If you deleted the doc then nothing is saved that is signature or lock related that would impact documents created later.