Even though this is probably a known issue, but I've been looking everywhere to find some information on this specific case and nothing surfaced. Here it is, the ERROR :-) (the big, bad SP error)I've been receiving "The data source control failed to execute the insert command." after filling out a metadata on a document within the document library with event handler (that fires up upon add item event). Cause: as part of my metadata I have a lookup column that is based on a list within the site and has "Allow multiple values" checked. Why it seemed that the event handler is cosign it? Well, I have another doc library with similar setup but there is not event handler behind it and it works just well. By the way it's not just a combination of Event handler and multiple choice lookup field, I also have a custom EditForm.aspx. Only after deleting my lookup field and recreating this field as a Choice field this problem has been resolved. And no, this is not SP1 environment. Hope it helps
SharePoint, event handler, Document library, custom edit form, multiple lookup field