Run Time Errors

Run-Time Error - Invalid procedure

This works for positive values of X but fails for negative values.

Sub NegativeX() 
Dim x As Double 
x = -10 
If (x < 0) And (VBA.Sqr(x) < 10) Then 
   'do something
End If 
End Sub 

You should use logical assignments as opposed to IF statements will also help performance (ie y = (x = 5)).

Run Time Error - 429 ActiveX component can't create object

If you are running 64-bit Office and there is a reference to a 32-bit dll.

Automation Error - The object invoked has disconnected from its clients

Automation Error - The callee (server [not server application]) is not available and disappeared; all connections are invalid. The call may have been executed

Cannot run Visual Basic macro because of a syntax error