One of my co-workers received the following error message when uploading a .pdf into a SharePoint Document Library
Form Validation Error
Please correct the information you provided by following these steps, then submit the information again:
The URL 'FolderA/FolderB/FolderC/FolderD/DocumentName #1234.pdf' is invalid. It may refer to a nonexistent file or folder, or refer to a valid file or folder that is not in the current Web.
Turns out this means:
You have an invalid character in the document name. In this case, rename the document without the "#".
Turns out :
There are lot of restrictions on document names (and url lengths) when uploading into SharePoint document libraries. I found an excellent summary of this information on Joel Oleson's blog File Name, Length, Size and Invalid Character Restrictions and Recommendations