I stored a copy of a PDF file in the Zotero structure (Attachments > Store Copy of File) and then viewed the PDF in Firefox 3 RC1. I realized that I had stored the wrong file (filenames too similar and I mixed them up), so I asked Zotero to delete the file. However, although the attachment was removed from the Zotero database, the physical file is still present on the drive. This, I suspect, has to do with the fact that the file is still being accessed (through the Acrobat plugin in FF) and so cannot be deleted. In such a circumstance, it would be handy for Zotero to close the tab or tell you that the file cannot be deleted until the file is not being accessed. The error should be available under Report ID 1919922667.