Government Watch Lists
Any due diligence investigation worth its salt should include many of these searches. The chances that a match will be made is extremely minimal, but the consequences of finding out you are doing business with someone on one of these lists could be catastrophic.
US Bureau of Industry and Security - Denied Persons List
Directorate of Defense Trade Controls - List of Statutorily Debarred Parties
Specially Designated Nationals List (US Dept. of the Treasury)