I had an interesting challenge last month and figured that this is a great one for my blog.   Being based in England, I had never come across this issue before so it was very interesting.  

I arrived on a customer site to help perform a Windows Server 2008 schema update and make sure that all was well and fix any challenges, should they arise.   We started by following Microsoft best practice to update the schema and when we started to run ADPREP /Forestprep, we found the following error in the event log:

Event Type: Error
Event Source: NTDS General
Event Category: DS Schema
Event ID: 1136
Date:  23/01/2009
Time:  1:02:38 p.m.
User:  NT AUTHORITY\ANONYMOUS LOGON
Computer: DC1
Description:
Active Directory failed to create an index for the following attribute.

Attribute identifier:
591789
Attribute name:
msFVE-RecoveryGuid

A schema cache update will occur 5 minutes after the logging of this event and will attempt to create an index for the attribute.

Additional Data
Error value:
-1403 JET_errIndexDuplicate, Index is already defined

The error appeared every 5 minutes in the event log, so I started to look around for this issue on the internet.  As it turns out, this is a known issue and I found the solution via KB932962 (http://support.microsoft.com/kb/932862/)

Basically if you are located at any of the following countries with the locale set to one of the following:

Arabic - Libya, Chinese - Singapore, German - Luxembourg, English - Canada, Arabic - Algeria, Chinese - Macao SAR, German - Liechtenstein, English - New Zealand, Arabic - Morocco, English - Ireland, Arabic - Oman, English - Jamaica, Arabic - Yemen, English - Caribbean, Arabic - Syria, English - Belize, Arabic - Lebanon, English - Zimbabwe, Arabic - Kuwait, English - Philippines, Arabic - U.A.E., English - Indonesia, Arabic - Qatar, English - India, English - Malaysia, English - Singapore, Spanish - Guatemala, French - Switzerland, Croatian (Bosnia/Herzegovina), Spanish - Costa Rica, French - Luxembourg, Bosnian (Bosnia/Herzegovina), Spanish - Panama, French - Monaco, Arabic - Tunisia, English - South Africa, Spanish - Dominican Republic, French - West Indies, Spanish - Venezuela, French - Reunion, Spanish - Colombia, French - Democratic Rep. of Congo, Spanish - Peru, French - Senegal, Arabic - Jordan, English - Trinidad, Spanish - Argentina, French - Cameroon, Spanish - Ecuador, French - Cote d'Ivoire, Spanish - Chile, French - Mali, Spanish - Uruguay, French - Morocco, Arabic - Bahrain, English - Hong Kong SAR, Spanish - Paraguay, French - Haiti, Spanish - Bolivia, Spanish - El Salvador, Spanish - Honduras, Spanish - Nicaragua, Spanish - Puerto Rico, Spanish - United States, Spanish - Latin  America, French - North Africa

You will more than likely run into this issue, so the fix is simple in most cases, just follow the KB article http://support.microsoft.com/kb/932862 and you will be fine :)

M