VBA MsgBox causes an erro - vba

Question

In my VBA project I have the occasional MsgBox pop up to notify the user something has 'Completed' or 'Updated' after a subroutine has run.
It seems to run okay without the MsgBox, but inserting one seems to give me an error.
Not sure if it's necessary to display the entire code here as it's quite big but at the end of a subroutine I simply want ...
MsgBox ("Completed")
which is followed by the End Sub
However when I run this and then click on OK on the Msgbox, I get a runtime error which on clicking DeBug, it highlights the End Sub.
Is there any reason why having this would throw up such an error?
Am I missing something from it?
Many thanks
Some of the code here
'Add unique data to new location
For i = 1 To UnqArray1.Count
rCell(i, 1) = UnqArray1(i)
Next
'Move Split Array into a new array
Set rTable2 = rCell
rng2() = rTable2.Value
'Filter into unique items
On Error Resume Next
For Each b In rng2
UnqArray2.Add b, b
Next
'Clear location
rCell.Clear
'Add new array to location
For i = 1 To UnqArray2.Count
rCell(i, 1) = UnqArray2(i)
Next
'Find the end of the category list
lastrow = Worksheets("CatMatch").Range("Q100000").End(xlUp).Row
'Sort alphabetically
Worksheets("CatMatch").Range("Q1:Q" & lastrow).Sort key1:=Range("Q1"), order1:=xlAscending, Header:=xlNo
'Copy it to CatMatch
Worksheets("CatMatch").Range("Q1:Q" & lastrow).Copy Destination:=Worksheets("CatMatch").Range("B15")
MsgBox "Completed"
End Sub

Answer 1

I can't reproduce your error, but you are almost certainly incorrect that it runs okay without the MsgBox. The problem is that the problem with your code is being hidden by On Error Resume Next in the fragment:
'Filter into unique items
On Error Resume Next
For Each b In rng2
UnqArray2.Add b, b
Next
Two comments:
1) Why not use the RemoveDuplicates method if that is what you are trying to do?
2) Your code is using the fact that a collection throws an error if you try to add a duplicate key. This is a valid use of On Error Resume Next -- but only if you turn it off when you are done adding keys to the collection. Something like:
On Error Resume Next
For Each b In rng2
UnqArray2.Add b, b
Next
On Error GoTo 0
A good habit to get into is to consider On Error Resume Next and On Error GoTo 0 as defining a block of code, perhaps even indenting the code inside the block as I did above. An even better habit is to not assume that only 1 type of error can happen. The above code is expecting that error 457 might arise (this is the error number corresponding to trying to add a duplicate key -- you need to search documentation to find it, or just run your code without the error handling and see how it crashes). Anything else indicates some other problem. To be maximally safe you can do something like:
On Error Resume Next
For Each b In rng2
UnqArray2.Add b, b
If Err.Number > 0 And Err.Number <> 457 Then
MsgBox "Unhandled error: " & Err.Number
Exit Sub
End If
Next
On Error GoTo 0
Doing this won't solve your problem, but should make your actual problem more apparent.