Why do I get 800A0BB9 / 800A1391 errors?

Typically, the following error will happen if you try and set invalid cursor or lock properties on a recordset object. 
 

ADODB.Recordset error '800A0BB9'  Arguments are of the wrong type, are out of acceptable range, or are in conflict with one another.

 
This is often because you used the VB "friendly" names for the values (such as adLockReadOnly), instead of the integer constants (which are the only values understood by the engine), without including ADOVBS.INC. So, a quick solution can often be to make sure you've included ADOVBS.INC. 
 
If you're using JScript, you might get the following error: 
 

Microsoft JScript runtime error '800a1391'  '<variable>' is undefined

 
In this case, you should be including ADOJAVAS.INC or a more efficient workaround. 
 
After that, you should investigate two things. One is whether you should even be using a recordset object; more often than not, the ADODB.Recordset object is unnecessary (see Article #2191 for more info). The second is whether it makes sense to include ADOVBS.INC / ADOJAVAS.INC, or to simply define the few constants you actually plan on using (take a look at Article #2112 for reasons to avoid using this massive file). 
 
Another thing that can cause this problem is if you use JScript / JavaScript and copy VBScript code over, e.g.: 
 

<script language=JScript runat=Server>      response.write("foo");  </script>

 
This should yield: 
 

Microsoft JScript runtime error '800a1391'   'response' is undefined