Just at first glance, I don't understand the InStr logic.. an AD Object can only be a member of one OU, so why all the AND logic? If you're going to match OUs using Case statements, create one for each exact match - or, as Shane pointed out - add delimiters to your queries. I wrap OU lookup with a leading "=" and trailing "," to avoid unwanted substring matches.

So - for the ER, match on "OU=ER," And "OU=Facility". Forget Computers in the case statements.. that's required by all matches so filter it earlier with a simple "If".
 Code:
If InStr($OU, '=Computers,')
 Select
  Case InStr($OU, '=Business Office,') And InStr($OU, '=Facility,')
  Case InStr($OU, '=ER,') And InStr($OU, '=Facility,')
 EndSelect
EndIf
The idea is to keep the logic as simple as possible.

Glenn
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Actually I am a Rocket Scientist! \:D