Why do I get 800A01FB errors?

Here are the situations I know of that can bring about 800a01fb errors. 
 

Microsoft VBScript runtime error '800a01fb'  An exception occurred: 'Open'

 
This error can happen when you use an ADODB.Recordset to retrieve a resultset, and the resultset is empty. Switching from ADODB.Recordset to conn.execute(sql) seems to alleviate this problem. 
 

Microsoft VBScript runtime error '800a01fb'  An exception occurred: 'Server.CreateObject'

 
If you are attempting to create an ADODB.Connection, ADODB.Recordset, ADODB.Stream or ADODB.Command object, this error might be caused by a corrupted MDAC. Update MDAC from MDAC Download Page. 
 

Microsoft VBScript runtime error '800a01fb'  An exception occurred: 'MethodForYourCOMObject'

 
If you try to pass an array to a COM object, you might see this error. See KB #217114 for information on passing arrays as arguments to VB COM objects.