This should give you the idea. Create a new project. Add a class mod. Put a commandbutton onto Form1 and add the following code to the class and form.
VB Code:
' Class mod code
Public Event BadMagic(strError As String)
Public Sub Hey()
Dim x As Long
On Error GoTo MyError
' change this to x = 1 /1 to see that event is only called on error
x = 1 / 0
MsgBox "Calculation a-ok"
Exit Sub
MyError:
RaiseEvent BadMagic("Divide by zero error")
End Sub
' form code
' need this to specify that class has events
Private WithEvents c As class1
Private Sub c_BadMagic(strError As String)
' just popup error msg
MsgBox strError
End Sub
Private Sub Command1_Click()
' creat new class
Set c = New class1
' call class func for processing
' if error in this func then c_BadMagic is called
c.Hey
End Sub