We have developed a set of metadata fields for the user to complete during the file upload process (MOSS).
What happens is that the user chooses Upload, then specifies the file on his system. Sometimes, when he sees what metadata data is required, he clicks Cancel, knowing that he cannot supply the data at that time. The file uploads, anyway, and is in the library without any attached metadata.
Our client finds this unacceptable, but I haven't found a way to cancel the actual upload when the user tells us he no longer wants to do so.