Why do I get 800A005B errors?

<object> error '800a005b'   Object variable or With block variable not set     or    Microsoft VBScript runtime error '800a005b'   Object variable not set

 
Make sure to use the SET keyword when creating objects (or calling methods that return objects). According to KB #243772, you should also register your component in MTS as opposed to running it standalone, though I've never been forced to do that (it's always been a different problem). 
 
If you have code such as the following: 
 

Dim objConn As ADODB.Connection  objConn.open "<connectionString>"

 
Make sure you actually instantiate an object using the NEW keyword, e.g.: 
 

Dim objConn As ADODB.Connection  Set objConn = New ADODB.Connection  objConn.open "<connectionString>"

 
If you have complex looping or if/else structures in your component, make sure your nesting is complete and you don't have any dangling structs that weren't picked up by the compiler. 
 
Do not attempt to use ObjectContext functionality within Class_Initialize() / Class_Terminate (see KB #250309).