Identify exactly where MERGE failed

I'm using MERGE to push DataTables into SQL Server 2008 tables and it seems like sporadically it will encounter an error trying to update the same rows multiple times.

When this has occurred I've manually dumped the DataTables to text files and inspected the tables & datatables looking for the dupes but I can't find any collisions.

Is there a way to identify exactly where the merge failed?

Edit: Issue turned out to be using an incorrect parameter in one of the开发者_运维百科 merges, I pass daily & total data which have different uniqueness criteria (daily might have today + tomorrow or today + yesterday depending on user timezones) which was breaking the update on sprocs where it was accidentally using daily data for the total updating.